I ask them questions. If they indicate that they are sinners who are eternally secure by grace because Jesus died and paid the penalty for all of their sins, I conclude that they are saved. If not, I am unsure as to whether they are a confused believer or whether they never were saved in the first place. In any case I then attempt to make sure that they now understand the gospel and accept it. It is conceivable that a person could understand the gospel, claim to believe it, and yet not believe it.
People can lie.
Some street people hear the gospel often. They learn the lingo. In order to get some handout they might parrot back words they don't believe. However, I think that this is a rare occurrence. I would only suspect that someone was lying to me if there were circumstances that made that likely.