Here is one method....
Take a small flashlight or penlight, turn it on and place it inside the empty humidor.
Turn out all the lights in the room (I would also suggest you do this in the evening as it will be darker) and see if you can see any light coming from the box.
Examine the humiodr closely, even lifting the lid slightly to see if any light shines through.
Also when you close it, you should here a nice swooshing sound of the air being closed into the humidor.
If you are worried, take a trip to Home Depot (or other store) and see what the have for window/door insulation, You can always add a thin strip to the seal if you are worried.
Hope this helps.
Al
Edit to add......it can take days for the humidor to stabilize. give it time. also get the beads and digital before you make any changes. I wouldnt trust those dial hygros for much.....