Evaluate a used aquarium when it is set up and running so the tank can be inspected for leaks and external signs of water damage. Often, aquariums are not running when they are sold; to inspect an empty used aquarium, inspect the seals and seams of the aquarium to make sure there aren't waters stains or salt build-up to indicate that the tank leaks. Make sure the seams are well-sealed with silicone.
Also check the surface of the glass for any cracks, chips or damage. Silicone can be used to patch these, but it is not recommended to purchase a tank with damage as a patch is unlikely to hold up over time. Scuffs and scratches on the glass are tolerable as long as they don't affect the integrity of the aquarium.
Verify that the upper and lower supports of the aquarium are not broken, as these pieces help with equalizing pressure inside and outside of the tank. The braces are replaceable, if needed.
Consider the age of the aquarium, but this piece of information impacts the aquarium's integrity very little. An older aquarium may be in better shape than a newer one. Also, make sure the custom parts included with the aquarium are in good working order.
Clean the tank and included parts after purchase, and then reseal the tank with silicone before filling it with water to ensure the tank won't leak.