Assuming you aren't sure of the manufacturer or model, I would definitely start with asking if it were real slate. I would also ask how many re-covers it has had.
It depends on what you're looking for, but in general, I would want to know the brand, model, size, and age of the table. I personally wouldn't spend money on anything that isn't a slate table, but again it depends on what you want. Obviously overall cosmetic appearance and structural condition are probably important. Beyond that though, cloth and rubber condition will let you know how much additional $$ you're going to have to put in to the table to replace those if they are worn out.
You may also ask if the table is currently assembled, or broken down, and if you're able to play on it before committing to buy.