I believe you can never really tell. I had a guy come up to me multiple times in Cherno asking if he could team up with someone and eventually after a zombie one-hit knocked me out I asked him if he could go find me some blood bags. At this point he started talking on voice chat and the guy truly sounded like a new player just looking for someone to play with. I couldn't sense any wrong-doing in his voice and he sounded like a genuine nice guy. After he healed me up I decided to let him team up with my group who picked us up just a little later. We then took him back to our camp, which admittedly was stupid but we trusted him. After giving him a sniper and some other gear we then set off to the NW airfield where we heard somebody dropping grenades on zombies and decided to go on a little hunt, however we lost sight of the players (like they knew we were there). We then decided to retreat in our cars and in suspicion of being followed we parked and assembled our group inside a large warehouse. After sitting there for a bout two minutes we were right about being followed and fire was opened on our position. One of our group members was shot but another managed to spot one of the enemies and take him out, which caused his friend to retreat. THe new guy told us where that guy was heading but instead of following we decided to escape. Sure enough we got away but just as we decided to log off, the same two guys from earlier appeared and killed us out of nowhere. Ofc my initial suspicion was that they were teleporting/map hacking but it turned out the guy who i grouped up with and played with for about 4 hours was with the enemy group and was feeding them our location the whole time! I was pissed off and now I rarely trust players, but I have a fairly large group so I still have a lot of team play. TL;DR: Played with a seemingly friendly player for 4 hours to find he was part of a bandit group and revealed our position to his friends.