A couple wears rings to signify they're taken. A gang member has a tattoo.
In Roman culture how did humans alter their appearance to communicate something to others?
Across social statuses what are the most common things to signal?
Rome certainly had a number of visible social markers, generally communicated through clothing. In the Aeneid, Vergil refers to the Romans as the gens togata (Ver. Aen. 1.282), so perhaps that is itself a good place to start. The toga, as the distinctive Roman dress, was the cultural marker of a Roman and at least by later periods a dress restricted to Roman citizens. One of Pliny’s letters relate a story of a senator sent into exile who appeared in the Greek pallium because as an exile he was no longer able to wear the toga (Plin. Ep. 4.11.3). One of Suetonius’ anecdotes in Divus Claudius suggests that this was a legal matter and not just a social convention. In his section on Claudius and the courts, Suetonius writes that, “when a small matter of contention arose between the counsellors as to whether it was proper for a man accused of being a non-citizen (peregrinitatis reum, lit. “accused of foreign-ness”) to plead his case dressed in the toga or the pallium, as if showing perfect equanimity to both sides, rather often he ordered him to change his dress, in accordance to whether he was being accused or defended” (Suet. Cl. 15.2). In this case, the accused man’s legal status affects whether or not he’s allowed to wear the toga at all. As a cultural symbol, the toga/pallium distinction reaches quite far back. Third and second century BCE Roman drama used Greek and Roman dress to distinguish between dramas set in Greece using Greek myths and those set in Rome using Roman historical figures and scenes. For comedy and tragedy respectively, Greek-clothed plays were palliatae and caligatae and Roman ones were praetextae and togatae.
Different groups were allowed to wear different types of toga, the differences between them clearly signaled by color and decoration. In the ancient world, both the Old Oligarch (the Pseudo-Xenophon of the Constitution of the Athenians), writing about Athens, and Appian, writing about Rome, use the ability to visibly identify social position from dress as a sign of order (or in both cases the inability to do as a lack thereof). Clothing was a clear way to signal social position. The toga praetexta, which had a purple border, was worn by children together with the bulla before the assumption of the toga virilis (for boys) or marriage (for girls). The toga trabea during the Augustan era was worn by the equites during the transvectio and later became a symbol for the equestrian order along with (also Augustan and later) the gold ring. The toga pulla was worn during mourning. Triumphing generals wore the toga picta. This is by no means an exhaustive list, but hopefully it gives some idea of the variety and specificity of some of the customs of Roman dress.
Women’s dress was different, but the demarcation of social status through clothing was equally codified. Aristocratic married women, matronae, didn’t wear the toga at all, at least by the later periods. They wore the stola. So when Martial starts an epigram, “Although you were not born from the upright race of the Fabians, nor are such as he whom Curio’s unpolished wife bore under the ruddy oak while she carries lunch to her husband as he plowed, but the son of a father shaved before a mirror and a toga-wearing mother, and your wife is able to call you wife, you permit yourself to correct my little books, which fame recognizes, and to carp at my jests…” (Mart. Ep. 6.64.1-7), you can be sure he doesn’t mean it as a compliment. Even beyond the gender-reversal happening in the other aspects of the opening, the mother’s toga is a clear marker of more than being in the other gender’s clothing. The toga for women was worn by prostitutes and adultresses only.
And of course once these rules exist, there exist ways to get around them or to use them to affect a different status than one’s actual. The Roman government legislated against the fraudulent use of status symbols, and this is one of those places where literature can sometimes offer a different view than what was legally the case. Martial writes a series of epigrams on the topic of Domitian’s 67 CE edict reviving the Republican era lex Roscia theatralis that poke fun at the different people trying to get around the new legislation and exposes some of the ways all those rules can be bent and broken. The revived lex Roscia reserved the first fourteen rows of the theater for the equestrian order and while by this time the military origins of membership into the equester ordo had somewhat fallen away in favor of whether or not a person had the equestrian census of four hundred thousand sestertii, it was still a closed group. It required free birth, excluding former slaves, even if they had the census, and having family in the order did not guarantee inclusion. The majority of Martial’s epigrams on the topic relate people who don’t have the proper census getting kicked out of their seats - “you bloviate about theaters and steps and edicts and trabeas and Ides [the transvectio] and clasps and censuses…” (Mart. Ep. 5.41.4-5) provides a fair list of all the various symbols of equestrian status that are variously put on in the many attempts to blend - but one particularly relevant one includes a slave attempting to pass as an equestrian in a way particularly reminiscent of an earlier book’s epigram about a former slave also trying to disguise servile origins through dress. In the Book 5 epigram, Euclides, “clothed in scarlet”, is attempting to keep his seat but betrayed by the key in his pocket falling out. In Book 2, the speaker calls attention to an unnamed man who “wears a dent in the front seats and whose sardonyx-adorned hand glistens all the way from here, and whose cloaks often get drunk on Tyrian dye and whose toga is urged to surpass never-touched snow and whose rich hair smells with the whole Marcellianus and whose polished arms shine from the plucked hair, on whose sole a lunate shoe-strap not even from yesterday sits, a scarlet shoe paints his unharmed foot and many patches spread over his star-covered forehead. Do you not know why that is? Lift the patches and you’ll read [why]” (Mart. Ep. 2.29). The man the speaker is describing is a former slave, using the patches to cover over the FUG on his forehead (an abbreviation for fugitivus and the mark of a runaway slave).
In the Roman world, distinctions of dress were used as a way to visually enforce status divisions, both culturally and formally.