most people won't fill a tank for you but some do. I usually have my 2nd hand tanks full of water when people arrive to see them. Then I simply use a big syphon hose to drain the water out.Is it out of the question to have the person fill the tank w/water to check for leaks before I buy it? I don't want to buy someone else's problem
You can check the silicon for damage. Look around the edges of the tank and see if the silicon is white or cream in colour. Also look for air bubbles in the silicon. If it has any bubbles or is white then try pulling at the silicon with your fingers. If it is coming away from the glass it will be easy to pull away. If this happens then the tank will need to be rebuilt before it will hold water.
If the tank has no bubbles in the silicon and the colour is clear (or black if black silicon is used) then it will probably hold water.
If the tank has been treated with any medication containing methylene blue, then the silicon will be stained pale blue. This does not affect the quality of the silicon. You will also be able to see air bubbles and any cream or white discolouration in blue stained silicon.