Answer:
It depends why you need to know.
If you are asking where is a good place a guy can masturbate that no one will catch him, that would probably be the bathroom or a room that no one enters, at an awkward time such as the middle of the night.
If you are asking how to masturbate without your wife/girlfriend being able to tell well that's easy. There is no physical difference before or after a guy masturbates other than his
1.personality and mood
2.ability and desire to have sex
so if you can perform again and act natural then no one will notice a thing.