Did people just assume everyone is freeman unless proven otherwise? So if a stranger comes to a village without chains he can just settle as a free man without any problems? I know free people often did the same job slaves did and skilled slaves could be relatively prosperous - so how do you differentiate between them? Also there were precedents like St. Paul getting an easier way of traveling through the Empire due to being a Roman citizen - so there must have been a way to prove you're a citizen.
It seems strange how structured society worked with slavery and citizenship without a clear way to tell people apart.
Some previous answers to your question:
I have a follow-up question. I've read the previous answers and they don't cover this. Who would accuse someone of being a slave, and what would prompt them to make this accusation? Who would be responsible for proving or disproving their claim?