"We've been expecting you Mr. Bond..."

I think most pre-16 year old teens even without highly inappropriate and unnecessary interference from adults of a sexual nature (such as in specific cases of child grooming and child abuse) pretty much discover they have sexual organs that strangely give them pleasure when stimulated a certain way. This kind of self discovery is normal. Masturbation obviously won't make you go blind as some age old made up stories will lead some to believe, probably made up and said in a vain attempt to stop rumbled teens by overly conservative parents from doing it despite maybe having done it themselves as teens.
Which is why I guess beyond any teenagers physiologically wired need to be highly curious about sex and having a just about containable higher sex drive during their years of puberty, the notion of fully grown and supposedly responsible adults indulging voyeuristic (as in more specifically secretly filming other people for their own sexual gratification) tendencies genuinely disgusts me. Whether the secretly filmed subjects are teenagers or adults. You are basically infringing on their privacy without their knowledge and active consent. That is if we were to turn the tables on such people for them to discover it was being done to them they would no doubt is some way feel violated themselves. Of course there is the exception of those who do actually get off on the thought of being watched without the person that's doing it knowing that they know they're being watched. This obviously doesn't include publicly accessible webcam broadcasts as they are specifically done for the intention of people to see with the subjects knowledge.

Mr Bond I presume?

I suppose we guys are simple creatures. For the most part its easy to tell what turns us on as we pretty much get an erection which again for the most part is a pretty clear indication. Notice I said "for the most part" as its NOT ALWAYS a clear indication. Obviously there is the exception of morning glory and the sometimes autonomous physiological response of getting an erection when we mentally genuinely aren't really actually feeling that aroused at all such as when being extremely tired after being awake for too long without sleep.
Having said that its also true that guys can sometimes be mentally aroused without any obvious physiological signs such as an erection too which is obviously a contradiction in itself but also true as far I'm aware.
However with women its a completely different story since they're great with double speak (if not triple or quadruple i.e. to the point where my mind would just shut down) and denial as if something was the truth. As far as sexual organs go its obviously not as obvious with there being no external phallus that becomes erect to give you an indicator. This also makes it much easier for them to deny things. However if we could attach some sort of perspiration and pulse rate monitoring device we'd certainly get a different picture as to what is truly happening vs. what they say.

Nice car...

It might not always be easy to see but there are other external tell tail signs that give an indicator. The size of the pupils and sudden changes in the size of them after exposure to the none contact stimulus. I won't say which way it goes as that should be obvious. Increased levels in perspiration in certain specific body areas, if not visually obvious as it is in some people it will still be detectable through touch.
Also as most will probably know by now, women when aroused produce a naturally occurring lubricating fluid. They become wet so to speak. Stimulating the clitoris with a dry hand isn't necessarily all that comfortable, however I'm guessing most women get around this when they figure out that they can use the wetness that is produced from the vagina to come from the opening when aroused to lubricate and make it comfortable whilst stimulating the clitoris by hand.
Also with any regular occurrence of masturbation it would be perfectly possible to make the clitoris less sensitive to the sensation of using a dry hand. Not to mentions that for some women it can be rubbed in such a way that allows for the folds and layers of skin to slide over it internally without any friction from the surface of the fingers to directly rub against the clitoris itself.
The smiles might one day become fake, and when they do its only a matter of time before they appear to look cracked and broken as the sadness grows and sets in.