As far as I understand, being a proper citizen of Rome gave you certain rights and privileges. Let's say I'm in trouble of some kind where my citizenship would be an advantage. How would I proove, or someone else check that?
Here you can see an excellent answer by u/astrogator: https://www.reddit.com/r/AskHistorians/comments/60nk4w/how_was_citizenship_verified_in_ancient_rome/?utm_medium=android_app&utm_source=share
Briefly summing up: you would have to defend your claim in court using witnesses and documents, such as a grant of citizenship.