Ruin her orgasm
This may seem cruel, but if she is really having one, she will be unhappy with you if you burst her bubble. If she isn't actually having one or about to have one, nothing will happen and there will be no harm done. It is probably the easiest way to tell; you know her body and her responses, so test her. Make it interactive, and you'll start getting the responses you want.
The male ego is a fragile thing, so it is important in your investigations to put aside your ego and deal with the facts. If she has constant problems having an orgasm, she doesn't need accusations; she probably just needs more quality foreplay.
The University of Chicago did a survey ("Sex in America," 1994) which reported that less than one-third of females always orgasm during sex, compared to three-quarters of men. The survey also found these statistics changed quite significantly when a woman's partner spent 21 minutes more on foreplay: 9/10 women always orgasmed with more foreplay. This stat says it all.
This situation can be difficult and embarrassing for you both, but it's certainly not the end of the world -- in fact, it's probably just the beginning. Deal with it in a positive way, and you'll both get positive results.
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