Can I tell if a woman is faking it? Perhaps not always, especially regarding the smaller ones. Can I tell if a woman has had a really intense orgasm? Yes, and always so. As a woman begins to orgasm, invariably her vagina begins to spasm (the more intense the orgasm, the more intense the spasms). This physical reaction as part of the female orgasm also serves the biological purpose of increasing the the likelihood of successful impregnation (the spasms "roll" toward the cervix, helping the semen along in their journey). If paying attention, the man will feel the spasms. When this occurs, mutual orgasm becomes more likely, as well.
As to whether or not it's somehow "bad" to fake one, I would say it depends on a a multitude of factors, intent being chief among them. I've faked a few in my day when my partner orgasmed, but I was just too tired or had already fired more rounds than the chamber holds, as it were. I enjoyed the experience, had generally already had at least one, and it seemed my partner at the time really needed the reassurance. If your partner is able to bring you to that moment, but just didn't this time (or won't, and it's not their fault), I'd say its probably ok.
If they don't regularly meet your needs, but can do so, then try to talk to them about doing the right things (especially when they do it right). If they haven't haven't for some time, then look within before looking without (trust me, you'll get better results). Once you figure out the problem (or if, no matter how hard you try, you can't), tell them what's going on, and explore your options.