if you can't see the tank in person, you'll just have to go with pictures, and descriptions. look for dents, and any specks of internatl rust. also, as whoever you buy it from if it's got the internals, as i hear that transferring those is a bitch. '00 750 is still a carby, right? make sure you get a carbuerated tank, as they differ from the injected tanks.
if you can see the tank in person, do all of the above, but bring a magnet and run it along the surface of the tank. if it's been repaired and bondo'd, there'll be places where the magnet won't stick.
might be cheaper just to get your's repaired. there's folks on the board who can fix 'er up right and proper for you.