Since I'm not an idiot, I've never been caught. However, at my friends house, while typing in newgrounds, I get a bunch of shit saying: dildos, lesbians, big tits, ect. I ignore it and proceed to NG to play some games. After awhile his mom busts in and asked me what's up with all the porn history on the computer I was just on. Luckily my friends brother saw that I was indeed on NG and said:"All he does is go on Newgrounds.".
Also some tips for not getting caught:
1) Never download porn! It won't take long for somebody to stumble upon your stash!
2) Make sure nobody can see you. Close the blinds, close the door (and lock it), fill any seems people could peep through, put the computer far away from the door, ect.
3) Don't disrobe, there is no need to have your pants to your ankles. Just unzip your fly and lower your under pants a bit. If you can manage to fap inside your pants, do so.