Overall appearance, clean or rust free. That will indicate recent use or care. Slight rust is not problem, it will sand off easily. Rust from soda cans will be a bigger issue. Avoid aluminum saw tops... period.
Contractor style with a belt drive and mootor hanging from the reare the the most common, but you may run into a hybrid with a motor inside the cabinet OR a direct drive. I have several older direct drive Craftsman which run on 220 Volts only and would buy another. They are my most used table saws.
Most contractor style saws have finicky fences, but nor all. Look for aluminum extrusions rails like on Rigid or Craftsman and other with a locking handle that square the fence to the rail and keeps it parallel to the miter slots ... a must. The fence is the "heart" of the table saw and is the most used accessory you will come into contact with while operating it. Other good rails are made of box steel, not angle, and also have a secure positioning and locking mechanism.
Look for at least a 1 HP motor drawing 12 AMPs on 120 Volts. Motors HP are over rated, so the AMPs rating is all you can use. A 3 HP motor rating running on 120 V is bogus, probably only "peak" HP. A true 3HP motor runs only on 240 Volts.
Blades come in all flavors and if they come with the saw, take the all. You can test and sort them out later.
You will want to get a new blade regardless, a 40 tooth thin kerf like Diablo, will do most any task. You really can't get by with only one blade, you will need about 3, 24 Tooth rip, 40 tooth general purpose and a 60 tooth cross cut for a complete range.
Table size and extensions are important. The open cast iron extensions that came with most Craftsman saws are a pain in the neck, since small piece fall through the holes, others get snagged on the cross ribs etc. Stamped steel extensions are easier to deal with and lighter, if that's an issue.
A rear mounted outfeed table is a real safety feature since you don't have to reach around or over a spinning blade to keep pieces from falling into the floor. Extra width can be gained by a wooden surround or adding more side extensions. Extra width is needed to support larger panels when cross cutting to make cabinets.
A large OFF/ON switch is a safety feature and that should be easily reached by a hip bump without taking your eyes off the balde and workpiece if there is an issue with a jamb.
Don't expect to get a used saw with all the above, but you can modify and add as you go along like many here have done, including myself. It's taken me 20 years to get the perfect saw setup for my shop. I have 3 saws mounter together, with 3 separate OFF/ON switches, 3 separate fences of different types that all mount on the same front rails for interchangabliity. Each saw wears a different type blade, and 24 tooth rip and a 50 tooth combination and one has a dado set permanently mounter with a sacrificial fence. This setup of course is not for everyone because of space limitations, but it's total cost is less than $2000.00 spread out over many years.
My current set up: