I could tell my female colleagues had sex with their partners by the minor changes in their behavior. The whole masturbation/sex thing is kind of complex because it involves instances such as addiction, refractory periods/physiological changes or some other controversial phenomena such as
Postorgasmic illness syndrome - with each of these potentially contributing to the person's "fitness". People are also frequently subjected to the effects of anosognosia for various reasons - such as a drunk person being unaware of or underestimating the changes in his/her behavior or a chronic masturbator not recognizing he keeps being numbed out and apathetic.