If you're going to jerk off, don't shoot off onto them. Hot liquid splashing on their skin will be noticed. If you absolutely must get your semen on them, deposit it somewhere else first, then transfer it.
Of course, all this goes out the window if they are drunk or otherwise under the influence. Passed-out drunks or drug-induced sleeps are generally less sensitive than usual, so you may have a little more leeway. Play it by ear, see how far you can go. Above all else, have fun.
