I recently had someone doing work in my basement and needed to run an errand and wanted to know whether the worker went upstairs into the living space. I was almost certain I could trust him, but still wanted to verify this. My solution was simple. I took a piece of scotch tape about 2 inches long and put it on the door frame and the door itself. Then I cut it about 1/8 of the way thru with scissors on both sides to make it easy to rip and also with the logic that if someone noticed the tape after they opened the door (which was unlikely because of it's position) then they almost definitely would not think to replicate the cuts if they thought to replace the tape with another piece. When I came home I shined my flashlight at the tape. It was intact and so was my trust.
Of course, you can also use this to insure that someone has come in. For example, if you need to make sure that someone that was supposed to come in to look at your house actually came in you would want to make sure that the seal is broken.
I'd note the importance of making sure that you are using a lightly adhesive tape like scotch tape because a really strong adhesive could obviously pull some of the paint off the door or frame.
In the first picture you can see the piece of tape placed on the door and notice the small cuts on both sides. In the second and third, you can see the tear in the tape.