Haven't seen an ad, or a website detecting I have an adblocker. The only thing I've seen is a website saying "an ad used to be here", but those might've just been placed behind the ad and not a sign of the adblocker being detected.
Youtube does sometimes detect you have an adblocker, but it's always fixed within a week. Even then, them purposely making your video load 5 seconds slower is less annoying than a 30 second ad.