A 20 gallon is not so large that's difficult to handle.
1. Put some newspaper on a level floor, put the tank on the newspaper and CAREFULLY fill the tank with water. Don't spill any water on the newspaper .
2. If after 24 hours or so and the newspaper is still dry and you don't see any signs of leakage, you are good to go.
3. Clean the tank with a vinegar solution, rinse well.
4. If the tank is leaky, you know what to do.
I haven't had a bearded dragon for more than 20 years but the one I had removed much of the aquarium sealant with his sharp claws. I wasn't able to even fill that tank with 3 inches of water before I noticed leaks on all sides. This was also a FISH designed aquarium. :eek: