Camera is probably best way, I don't have one either. Those little magnetic wires with balls on top that you stick to your hitch and to the trailer would probably work pretty good too.
What I do is backup until I'm close (like within a foot or so), then stop, get out and walk to the back of the truck. Note how far the ball is from the trailer, then get back in the truck and leave the door open. Then, I look straight down and pick a spot on the driveway and use that as a reference point to see how far to back up. Works almost every time (I tend to be conservative and stop a little short sometimes).
Note: It also helps to keep the trailer tongue close to the height of the ball so that it is likely to hit the ball first before getting to some other part of the truck that you don't want to hit (just in case you do back up a little too far).