Need help differentiating pre cum and cum (read)

hey all. this is kinda weird but hopefully you’ll get where i’m coming from. So Me (19M) and my partner (18F) have been having sex a lot recently, and she’s the first person i’ve ever had sex with. We’ll have sex for a while (2, 3 hours) and i won’t finish. Now obviously when i masturbate i can see and feel the difference between pre cum and cum, but when we have sex, sometimes i get scared that what i thought was just pre cum was actually just me finishing prematurely. Does that make any sense? Having sex just feels so different than being by yourself. I don’t know. I’m also on Zoloft which sucks. Let me know if anyones experienced the same. I feel like i would be 100 percent certain when i’m orgasming, right? It just hasnt happened yet while having sex which is weird.