Smaller tanks I usually have no issues with snapping them up if I have a need. The inspection is pretty easy: Chips, outter silicone beads (the part that you can feel on the inside) and making sure the trim is intact around the perimeter. Small scratches aren't a concern, but if there's a nasty haze it's a sump at best. If REALLY cheap, or even cracked, some make very inexpensive glass tops at the right thickness.
I would have 2 concerns with former SW tanks.
Coralline algae that's allowed to build up in spots for long enough will etch the glass surface, leaving slightly hazy areas behind. They usually aren't in viewing areas so the haze isn't the issue, but those areas now become a haven to some film algaes and become a real bugger to scrape clean, requiring nothing less than a fresh razor blade.
SW itself, MH lighting and even T5's can do some real damage to the centerbrace on larger tanks. If it looks severely faded, you should be at least doubtful that it would hold up well over time, especially after draining, drying, and re-filling. It's likely become somewhat brittle and may be on borrowed time. Yes, if it was built with a brace, it needs to be there when the tank is full.
In a nutshell, if it wasn't a SW tank for some years, I wouldn't expect more than any other inspection would detect. If it's been SW for a bunch of years, the inspection needs to go beyond the superficial aspects.