How can you tell if your partner is sleeping with your friend?

a few months ago i befriended this girl who became very friendly with me very quick we got on very well my partner already knew her from childhood a couple of weeks into us all hanging out together i noticed her number in his phone under a different name when i questioned him he said it was because he needed a lift somewhere then i found some of her make up in his car but she swears she left it there when i was with them but im sure it wasnt ive also found more things of hers in his car then i found out he was staying at her house for 4 days when i confronted him he said it was because he wanted to split mine and her friendship up as he was jelous when i confronted her she denied it. Then another friend saw them together in a pub with other people too but no one invited me when we used to hang out at the start they would always talk about the old days and childhood people but after a couple weeks whenever we was all together they would hardly ever communicate with each other which i found strange there is loads of other little things ive noticed eventually i accused him and sat him down and got him to admit they were sleeping together again she denied it then he took it back and said he only said it cause i pressured him into admitting something he didnt do. When i accused her she has now blocked me and he is still swearing he never slept with her im so confused am i being paranoid?