Just pay attention to your snakes behavior. Usually they go still for a minute before doing it. They also straighten out as much as possible (at least in my and someone else's case) and while peeing don't seem bothered by your movements. What I mean by that is...a while ago I was watching TV on my couch with the snake exploring on me when he straighted himself out and became still for a minute. When I realized what he was doing, I lifted his tail and jumped. Usually the snake would have snapped back, but nooo...it's almost like he knew he was pissing ON me and was enjoying my reaction lol.