No, he cannot.
Unless he has you wired up with electrodes and vaginal probes à la the empirically minded doctors Masters and Johnson in their spooky Frankenstein sex lab. Frankly, I'd rather be kidnapped by aliens.
Which is not to say that you should be faking orgasms. What's the point of that? Sex is supposed to be about pleasure. Plus, I'm presuming that you actually like the guy you're bedding. (If you don't, you shouldn't be intimate with him.) It's not nice to lie to people you like.
An attentive guy, however, should be able to tell when you're sexually aroused. Sexual stimulation increases blood flow to your genitals. Your heart rate and blood pressure go up; the head of your clitoris swells (although it may also retract a bit up into the surrounding folds of your labia minora, which can be a bit confusing for a guy who's expecting erectile changes like the ones he sees in his own penis). There are also changes and swelling inside your vagina that produce lubrication.
The thing that's always enraged me about the porn industry is that it's so obvious that most of those women flailing around in the throes of feigned ecstasy ("Oh, baby! You're so good to me!") are not sexually aroused in the slightest. And since the availability of more wholesome sources of sexual information in this culture is so abysmal, this is how many guys form their notions of female arousal.
No wonder it's so easy to fake them out.
Patrizia DiLucchio has been a nurse, model, topless dancer, criminologist, postal clerk, health policy analyst, Hollywood executive, editor, writer and index entry in The Joy of Cybersex.