Sadly, rear suspension takes a huge beating and it is generally the most overlooked during regular maintenance. There are almost as many bearings or bushings there as there is in any one area of the bike. The shock generally has one bushing or bearing at the top. The clevis generally has between 5 and 6. The swing arm generally has between 3 and 6. Manufactures use to provide us with convenient Zerk fittings for easy lubrication. Not any more... In fact, most bearings that come from the manufacture installed on a bike are under lubricated
So you find a cool bike on Craigslist and it looks great with those new plastics, tires, and grips. How can you tell the condition of the rear suspension?
1. Sit on it and bounce up and down. I have had multiple bikes freeze while on the way back up. It's cool if you want a low rider but not so good out on the trails
The other thing is feel if there is and dampening or rebound. If it feels like you are sitting on a bouncy spring (Boing boing boing) that means that you likely are 
.2. After bouncing it up and down several times, put the bike on a bucket or stand so that the rear wheel is off the ground. Look carefully at the chrome shaft. Is there rust or pitting? If so it's time for a new one. Is there oil weeping onto the shaft? It's time for a rebuild. Is the rubber stopper missing or cracked? If so you will need to disassemble the shock to replace it and likely rebuild it while you are there.
3. While on the stand, grab the rear wheel and pull up on it. Can you wiggle it up and down? You should not be able to feel any free play. If you can this means that the bearings or bushings are shot in your shock, linkage, or swingarm.... To replace them all yourself can cost you a easy 150-200 bucks
4. look at the insides of the linkage and the bottom clevis of the shock. Do you see unusual wear marks? If so this can be caused by a number of factors... All of them will cost you money to fix.
5. Your shock likely has one or more adjusters on it. Take a screwdriver and try to move the adjusters. Do they click fairly easily and click into place? If not you have other issues that will need to be addressed.
There are not the only things to look for but these simple inspection tips will help you to see if you will need to spend additional money before you can take that bike out and ride it.