At least in Classical Athens during the fifth and fourth centuries BCE, which is the Greek polis and period for which the most extensive sources have survived, it was impossible for an observer to consistently tell an enslaved person from a citizen by their appearance alone. Indeed, this very fact seems to have greatly frustrated and perturbed some wealthy slave-owning aristocrats who wanted class distinctions to be more visually evident.

Sometime in around the late fifth century BCE, an anonymous disgruntled reactionary author, whom modern scholars have nicknamed the "Old Oligarch," wrote an essay titled On the Constitution of the Athenians. In this essay, the Old Oligarch famously complains that, in Athens, it was illegal for a citizen to hit an enslaved person who did not belong to them, because it was impossible to consistently tell a citizen and an enslaved person apart solely by their appearance and, if it were legal to hit another person's slave, then citizens would end up mistaking other citizens for slaves and hitting them on a regular basis. He writes in On Constitution of the Athenians 1.10, a translated by E. C. Marchant:

"Now among the slaves and metics at Athens there is the greatest uncontrolled wantonness; you can't hit them there, and a slave will not stand aside for you. I shall point out why this is their native practice: if it were customary for a slave (or metic or freedman) to be struck by one who is free, you would often hit an Athenian citizen by mistake on the assumption that he was a slave. For the people there are no better dressed than the slaves and metics, nor are they any more handsome."

The Old Oligarch, of course, harbored a deep-seated hatred for Athenian democracy in general, so one should take his words with a few grains of salt.

It is also worth pointing out that, although it was impossible to consistently tell a person's status based solely on their outward appearance, there were certainly some visual clues that a person could use to make an educated guess.

For instance, one could expect that enslaved people who were forced to do hard, manual labor outside might have rougher, dirtier, or less kempt appearances than many citizens. Most citizen men who were not wealthy, however, had do a significant amount of manual labor outdoors for their survival. Meanwhile, a large number of enslaved people, especially enslaved women, worked mostly or exclusively indoors as domestic servants. Thus, a person having a rough, dirty, or unkempt appearance was not necessarily a reliable indicator that they were a slave and having a crisp, clean appearance was not necessarily a reliable indicator that they were a citizen.

Enslaved people and citizens may have had some slightly different tendencies in their grooming and clothing. In Attic vase paintings, adult enslaved men are more likely to be shown as clean-shaven than citizen men and are often depicted wearing the exomis, a kind of simple, off-the-shoulders tunic. Citizen men also sometimes wore the exomis, but enslaved men may have worn it more often.

Some enslaved people in Classical Greece, especially those who had previously tried to run away from their masters and been captured, bore brands or tattoos on their skin to mark them as enslaved. For instance, the Athenian comic playwright Aristophanes (lived c. 446 – c. 386 BCE) references a tattooed enslaved man in his comedy The Birds, originally performed at the City Dionysia in Athens in 414 BCE, lines 760–761.

In addition, some masters also forced some or all of the people they enslaved to wear chains—again especially those who had previously tried to run away and been captured. In his comedy The Frogs, originally performed at the Lenaia in Athens in 405 BCE, lines 1508–1514, Aristophanes portrays the god Plouton as threatening to punish Athenian politicians with tattoos and chains, as though they were captured runaway slaves. Not all or even most enslaved people, however, seem to have born these sorts of markings.