I know i shouldnt trust them, but how do i know?

Once i was eating dinner and someone left an IM on my screen, but i didnt know that screen name. so i figured it was one of my friends who just got a new sn, but when i asked all of my friends if they were that person, they all responded no, and i can tell when theyre lying. so that person came online and i started talking to them and asking them who they were, but they wouldnt answer me and they kept stalling. so then when we FINALLY got on friendly terms, he said that my other friend who was IMing him told him where we lived. I then asked her if she told him and she just asked if he went out our school (the name of the school too.. nice job...) she also said that he knew her name and she didnt tell him. He said to her, Is your name ________? and she goes, ummmmm why... it is really creepy but when we talk he acts just like a normal young person who acts like he's 13 (like he said). i knowi can't trust him/her at all, but is there anyway to know when hes really telling the truth?