Leave a $20 bill on the floor near the entrance. If anyone comes in, they'll surely pick it up. You can tell immediately when you return and open door that someone who was unwelcomed has been in and you can take necessary precautions to make sure they are not still inside.

Of course, it is probably a good idea to place the $20 bill so that you have to be inside to see it. You'd hate to invite someone in just to get it (i.e. door-to-door teenage magazine salesperson peers through window in door and sees $20 and no one comes to door . . .).