You can't. No one can. Non-magical people can't see them. But what they will do is basically paralyze you with depression. You won't move because you are unable to, and they will get closer and closer, sucking at your soul until they are so close to you that they will eventually suck it out of you, permanently. It's called the Dementor's Kiss, and you must take all caution if you begin to feel this way.