Maybe it's because of the size of my cock and just how very deep I must go just before shooting my loads; but, more guys than not have said they could feel me cumming in them. I've measured the amount, and my average load is about twice that of most men--and, the past few years, it's been more than that; so, the size of my loads is probably a factor, too, even, though I haven't shot hard for 20 or so years. Just from my own experience, it seems that guys who are slim or more fit and guys through their 20s have been more likely to tell me they could feel it. When talking with a man before meeting him, when he says he always feels loads shot inside him, that becomes a major reason for wanting to meet and fuck him.
As far as behavior goes, some guys have said my cock gets slightly thicker after we've played awhile and kept my cock hard. It is unmistakable when I know I'm close to cumming because I do fuck harder, my cock get rock hard, and I simply shove my cock hard in as deep as possible at the point of orgasm, not to mention I'm usually loud then, too--so, men always know for certain when I'm cumming in them, whether they can feel it or not.