Sumptuary laws, perhaps surprisingly for something as omnipresent as they indeed were (in all of Europe, and not just Italy) are a bit of an understudied area. This is in part due to the fact that they were very localized affairs subject to changes over the years and centuries (making broad and consistent surveys difficult to construct) but also because being social/societal regulations they very often fell in the camp of Canonical/Church Law, perhaps surprisingly to our eyes but ultimately in keeping with the broader regulation of social customs as Renaissance Italians understood them. This means we have little to nothing on sumptuary enforcement to go for in the otherwise extensive legal archives of local governments in Italy, and instead have to dig through ecclesiastical records which while normally just as meticulous, might have important information broken up between records held by individual parishes, episcopal seats, and other religious institutions. This means that ultimately, matching decrees and their enforcement a difficult and time-consuming task. We can, however, point to a few macro trends that have been observed in the development and enforcement of sumptuary laws:

• Excluding ancient Roman sumptuary laws, regulations on dress and appearance in Italy actually start appear in the thirteenth century, long before the Renaissance. Some hypotheses point to the fact that these laws sought to limit the ostentation of the old aristocracy in urban spaces as the working class and bourgeoisie earned their political enfranchisement via conflicts that often turned bloody. Sumptuary laws in this context act as social equalizers, signaling that the privileges of the aristocracy have been curtailed, and imposed some form of equality in the urban “Commune.” This could be adopted even in those societies which moved in the opposite direction, notably in places like Venice, where every turn towards oligarchy was accompanied by strict sumptuary laws designed to obfuscate the intentional construction of an increasingly unequal society.

• It follows that themes of conformity also translated to using sumptuary laws to regulate how certain social groups or economic operators dressed: bankers, merchants, lawyers, and other social categories could impose conforming dress to signal to each other and to outsiders the social and economic category to which they belonged. This means that the tight-knit social communities of the Italian cities could largely be expected to self-regulate how they presented themselves to each other and to outsiders, but also means that over the centuries Sumptuary Laws naturally departed from their "Social Equalization" role and evolved into laws in place to identify distinct, and often privileged, social classes (and to browbeat less privileged classes or people with “outsider” status, or to exert forms of societal control on groups like unmarried women).

• Social pressures and narratives also worked to enforce and rationalize sumptuary laws, as perceptions arose that ostentatious dress or flaunting of wealth represented unproductive use of money that could instead have been more productively invested in the economy or shared with the family (this is particularly true for sumptuary laws targeted at women). This also translates to decrees of sumptuary restrictions implemented in periods of penance or periods of mourning. Sumptuary restrictions were a way for leaders to regulate the local mood in the wake of wars, plague, or natural disasters.

But apart from social pressures or the practical societal function of sumptuary laws, you might still insist on your original question: how were Sumptuary Laws enforced in concrete terms?

There's a whole school of thought that would answer, "They Weren't," and claims that social pressures born out of a desire for conformity in a society that placed enormous value on social capital were pretty much the only thing keeping people in line. However, while it's probably true that social pressure was responsible for the bulk of enforcement, careful digging does reveal a few other distinct methods of enforcement:

• Some sort of monetary fine could be imposed on people violating sumptuary laws which if not coupled with social pressures, means that it was fundamentally a wealth tax (thing is, we do know it very much was coupled with strong social pressures to conform, meaning that it really wasn’t envisioned as a form of taxation and was truly meant to be a penalty — although there were exceptions: the city of Siena, for example, had a weird pre-payment system which hints to some conception of leniency for the very wealthiest)

• Another (fairly clever in my view) way to enforce sumptuary laws was to have tailors and jewelers swear before local authorities that they would not create clothes or jewelry which violated sumptuary laws. This could be done either as a one-off upon a new sumptuary law's proclamation, or individually when new guild members were sworn-in. Either way, this method additionally served to involve guilds in the "validation" of sumptuary law and augmented their legitimacy to the community (even though the very wealthiest and well-connected could alwayshave their clothes made elsewhere).

• A third way to enforce sumptuary laws is to bar people wearing offensive clothing from entering public buildings or, especially, churches (recall that sumptuary laws were, most times, components of Canon Law). Thus a community leader or church functionary, posed at the door of churches on feast days could deny entry to people dressed in too ostentatious a manner. This kind of control would be very effective, as religious holidays and feast days were an enormous component of social life, and these were the very days when public ostentation was most likely to occur.

• A final thing to consider is that by the mid 14th century, another sort of sumptuary law appeared which didn’t explicitly ban wearing certain clothes, but instead decreed what kinds of clothes were acceptable to wear. While this isn’t really a method of enforcement, it does represent a codification of what society believed an “acceptable” outward appearance looked like for a law-abiding and civic-minded burgher.

I suspect your question is born out of disbelief that people could effectively police each other’s appearance. And you’d be right to be perplexed: there is little evidence to show that authorities were actively counting the pearls in a women’s circlet to see if they were too numerous (or interrogating women who were in violation if they happened to be a notary’s wife) or measuring the hilt of a man’s dagger (both actual things regulated by sumptuary laws). Enforcement was usually done by peers, and when authorities cracked down it usually occurred during religious holidays, when whole communities piled into religious buildings and might have an incentive to one-up each other. It also goes without saying that almost as soon as sumptuary laws were promulgated, people got to work trying to loosen them asking for carve-outs and exceptions. For particularly influential communities or groups of people, exceptions to at least some restrictions were very often granted: judges and other public officials, “Cavalieri” (itself a vague term like English Knights, be they real or ceremonial) and especially married women (but almost never unmarried women, and at any rate only those of the highest classes — perhaps an accepted workaround for aristocratic families to flaunt their wealth). These exceptions mirrored people's privileges in wider society: community leaders might have sumptuary exceptions, and they might also have a privileged place to sit in the local cathedral, or even their own chapel, or their social category might have a particularly ornate guildhall. In other words, sumptuary restrictions and concessions existed within a wider social narrative.

But on the topic of carve-outs and particularities of various sumptuary regulations, it's important to keep in mind that different communities engaged with them in different ways: the Venetians largely resisted the urge to carve out exceptions, and their sumptuary laws were themselves unusually austere and strict. Similarly largely lacking in carve-outs were the Milanese, however for a very different reason: Milan had very weak sumptuary laws to start with, and were among the last cities in Italy to implement them.

Every sumptuary restriction, and ensuing exception, represented a deliberate decision by local decision-makers. While these laws were initially envisioned as a social equalizers, they quickly became instruments of conformity, subject to contemporary notions of modesty, and eventually instruments of social differentiation and markers of class consciousness.