I got extremely lucky with my current partner (also first PIV orgasm). I typically have to be on top, but when I am if he pulls and pushes on my thighs/hips that usually causes it to happen. It's a suuuuper intense climax (whole body) and usually lasts longer than clitoral with out the spasming. One indicator for me is that i will get a tingle up my spine.

Good Luck!