I feel his erection increase, the texture on it becomes more defined... (veins, ridges etc) the moment before he cums especially, the throbbing... I can definitely feel it the most with my mouth, second with my hand, in my vagina I can feel him getting "harder" and the throbbing but I can't say I have actually felt his ejaculate if he cums inside me... (kinda HOT now, typing this haha)