Like others said . . . you usually can tell when he cums by how he moves, his breathing, vocal sounds, etc. Beyond that, there's a lot of individual variation.
If he continues with vigorous, or even moderate, strokes during his orgasm and ejaculation I won't feel much, if anything, except the stroking.
If he starts his climax with a forceful cum-thrust, the hurt may keep me from feeling the twitching, jerking, or pulsations of his erection. (If the pain isn't too much this can actually be a turn-on: he thrusts; I flinch and grimace, whimper or even yelp a bit; he holds me firmly or pins my body in place so I can't squirm away until he has finished the insemination; I may clutch the blankets or kick my legs as I try to escape the discomfort of being deeply impaled. But it can be a fine line between "hurts so good" and "hurts too much".)
If he slows down, or even stops the fucking motions, I might feel his cock head jerking, or shaft pulsating inside me. By far, my favorite is to be fucked in missionary position, and just before the guy cums he lays out full-length on me, pushes to full depth, relaxes, and lets his orgasm happen. I feel his penis twitching inside, his body shuddering against mine, and my hands on his butt cheeks feeling the muscle contractions as his seed spills into me.
If his girthiness is just right, I may feel the shaft pulsating against my stretched anus and sphincter.
My normal body temperature is on the low side of normal (around 97 - 97.5) so I occasionally (but rarely) feel the warmth shoot through me with his first squirt. More commonly (but still not often) I feel the texture change inside me as his cum spills freely into my rectum. A few guys will stay hard inside me for a few strokes after they have finished, and I can tell that I have been spermed.
It is a unique and very satisfying sensation to know that a guy is cumming inside me! In addition to the physical sensations, there are mental sensations of knowing that I have been taken by a man and used for his sexual pleasure, and I have helped give him that pleasure. Sometimes I think about the one virgin girl I deflowered, and imagine that she must have felt similarly: it's at least a little uncomfortable, but it's a strangely pleasurable kind of discomfort and satisfying to know I am giving pleasure to my partner.