Is there a way to know if my boyfriend cums in me ?

Ask him: Apart from the obvious - asking if he ejaculated- check the vaginal fluid. Smell your hand. Semen and vaginal discharge have different texture and smell.

No way for sure.: It is possible that your boyfriend ejaculates inside you yet no semen is visible, so there is no real way of knowing. This exemplifies the importance of using condoms during intercourse.