how do I know if it's semen or my own wetness on my clothes??

Today my bf and I were fooling around and I started straddling him and rubbing against him. All was fine and dandy until I notices there was something wet on the crotch area of my leggings. I can't usually tell when he comes, but I know it happens sometimes when we do this, and I'm on my fertile period so I'm worried about the off chance that I might get pregnant because of this. Also because I was on my fertile days and also super horny, my wetness soaked through my panties, so it might just be that, but I'm still worried. Is there a way to tell the both apart on my clothes?