During a 3sum, I was fucking a girl from behind and she said to the other girl in front of her "oh my God, he's so hard". It was when I was on the edge of cumming.
I looked it up, and yeah, apparently a guys cock usually gets harder just before cumming. I suppose it's natures way of ensuring all that spunk gets out easier.
Also I think the laboured breathing thing is a giveaway.