if you're not talkign about an already setup tank but an empty one check the seals maybe sure they are all tight and not coming off the glass. if the seals are hard then thats not a good thing either they would have to be replaced which is actually an easy thing to do. all you'd have to do is go to home depot and purchase 100% sillicone GE #1 or DAp 100% sillicone and apply it with a calking gun.
make sure there are no cracks or chips and that inbetween the glass there is no black mildew, this would indicate the need for redoign the joins, same as redoign the seals.
glass doesn't usually wear out unless subject to high heat and vibration which most aquariums are not, so all that needs to be done is joints and seals if you want to be 100% sure it won't leak.
i'd buy a used older tank and for an extra $20 for the security of knowing it won't leak around the glass is just as good as buying something new.
i was considerign buying a used 150 gallon with 20 gallon sump and stand for $150, the aquarium was 22 years old and had 3 days down time in those 22 years. only thing that stopped me from gettign it was i wasn't goign to set it up right away and my parents wouldn't let me store it in the basement
its long been sold
the dimensions were 72x18x28 would have been a sweet reef when i would have eventually set it up