Depends
Inspect it closely, ask questions like has it always been in use? If stored, where.
Its not good to store glass in the cold. Can cause the silicone to get brittle and ultimately leak.
Just inspect the seams closely, preferably filled to see that it holds water.
But you can quickly tell if a tank is worth getting. Does the glass have scratches? Glass is harder to scratch, so if it has scratches you can assume that's it's old has been used a lot. Same with the frame. If its ragged looking, cracks and such will give you clues of age of the tank.