In all seriousness, it really comes down to assessing whether you feel the other players are being genuine or not (as town has little/no reason to lie, whereas mafia has to constantly lie). There are lots of ways to go about doing this. For example, if somebody spends all of their time pushing somebody and that person happens to be mafia, whoever was pushing them is PROBABLY town. If the mafia was defending somebody, they have an increased likeliness to be a teammate.
Disclaimer: this is a very, very basic analysis of how the game works, and as you play a large number of games, you'll see it's not as simple as I've illustrated here. You also will find a method that works for you. Some people put a lot of stock into who voted for who ("vote count analysis"). Others don't care at all/don't understand how it's useful.
At the end of the day, this is just "guess who?" for adults where a quarter of the game is lying about what their character looks like. Most players have games where they're right a lot, and they usually follow them up with games where they're wrong about absolutely everything and are a complete detriment.