How did I get a Roman passport? How did I prove I was a Roman citizen and not a runway slave? Could other cities in Rome ever find out what I did? How did neighboring countries deal with immigrants from Rome?
A similar question was asked a while back:
. . . where u/Aithiopika and I dug into this topic
Of course, there's always more that can be said . . . take a look at the prior discussion and see if there's something that might be expanded on.
Note that there was no "Roman passport" -- proving "who I am" would be a matter of my references, eg "where you say you're from, who you know" and so on. There were documents that people might possess, a manumitted slave would have a document to that effect, a veteran soldier would have a kind of military diploma, but there was no one identity document. With limited media, it would have been harder than it is today to convince someone that you were from someplace other than where you actually were from-- you would have limited access to the information you'd need to plan a backstory.
Rome had deep social networks, and trying to hold together a story would be hard. . . as u/Athiopika noted:
"Roman society has been described as a face-to-face honor society. Social ties, ancestry, relationships, etc. are very important in establishing one's public identity and status. If a stranger shows up in, say, Pompeii claiming to be a local citizen, people will want to know why they've never heard of you or your family."
. .