What kind of truck do you have, and does it have the factory towing mirrors on it?
Also depends on what you mean by "see behind me". Even massive, extended towing mirrors don't allow you to see directly behind your trailer. You will learn to keep track of traffic around you, and as MarkTwain said you can move back and forth just a little bit to see if anyone is tailgating your trailer - just a little bit of movement, you don't want to start really wagging the trailer!
When you drive around not towing, get into the habit of using side mirrors on your truck or car. Learn what they show you, and what they don't.