Here are few simple tricks you can do:
- Put a small hair between the door and the frame, but visible so it is exposed slightly outside. If you can see it is dislodged already, the door has been opened. If not, then it has not.
- Get a small piece of paper and tape it to your door or door frame, in such a way that you can put it in a position. When the door is opened and closed again it goes to a different position.
- Put something very close to the door. but just close enough so the you can slide thru the door without moving it. If he opens it completely the object will be moved.
- Get security camera with motion detection.
I hope you try this useful tips at home.