For my particular implant installation, it would be hard for them to tell. I guess I am lucky, but there is no tubing that can be felt (except for maybe a subtle firmness in my inguinal area) going up to the reservoir...but nobody except for me and my surgeon would be able to feel that. I have no tubing at the base of my penis shaft.
When erect, it is like a super hard natural erection, so they just enjoy that. When my balls slap onto their butt cheeks during doggy style, they would have no sensation that the pump is there
My pump is pretty far back (i.e., quite behind my balls), so it is possible that they might miss it when grabbing my balls; though, a perceptive women with good hands likely will detect it when grabbing my balls. Visually, however, even when doing a THOROUGH visual inspection of my penis and scrotum, they will NOT be able to tell.
Agreed that you can simulate the natural erectile process by pumping half way up (15 times) during the end of the dinner date (I am still sort of flexible at 15 pumps). Then, during foreplay, it would be possible to do the extra ten to fifteen pumps while kissing her thighs and legs just before penetration.
The big give away is that you are still hard after orgasm. With the quadmix injections, that was also the case for me, and I think for some guys (who are super sensitive to Viagra) , they also might stay hard after orgasm. So the main time that you would need to explain why you are different would be when it is all over--but, then again, she might be so overcome with hyperventilatory orgasmic delirium that she might just be unaware as you pull out
All this being said, some men have told me that "if you are having sex with the woman, you should probably tell her, just to establish trust." The woman might view this as a deception if you do not tell them; though, I could Imagine that younger guys might want to keep the implant a secret.