I agree that it's pretty easy to tell if you're in an oral or digital situation. I think I could conceivably be fooled in a straight missionary-type situation, though.
This topic reminds me of the deathless phrase in an old National Lampoon essay:
"Is It a Real Orgasm?
The honest female orgasm is three to fifteen rhythmic contractions of the outer third of the vagina at .8-second intervals (the contractions follow the beat of the song "Surfin' USA"). Unless these contractions occur, you can regard her groaning, moaning, clawing, kicking, begging for mercy, and shouting filthy religious epithets as bargain-basement histrionics. "
For the rest of the (extremely funny) essay,
Click here