Depends on what you are wanting to do with it.
If you want a rifle that you can immediately use, then take the following tools with you when you go used rifle shopping. A bore light (even a small flashlight reflecting off a white piece of paper will do) and a cleaning rod or boresnake (I prefer a boresnake because they are handier and don't run the risk of causing wear on the muzzle), and a small mirror if the rifle you plan to buy doesn't allow you to sight down the bore from the breech.
Inspect the rifle for rust or pitting. You can usually get a bit dropped off the price if there is either.
Run the boresnake through the bore then inspect the barrel from both the muzzle end and the breech end if possible. You want to see strong rifling at the throat and no interior pitting or "frost" in the bore. If you do see "frost" you can haggle the price down. The bore should be shiny, and all bores are shiny if they have oil in them, hence running the boresnake through before examining.
Check the action for wear, any chipping on engagement lugs, see if the safety works. If there is damage to the action, do not buy the rifle. It may be safe to shoot for the rest your life, but it may fail immediately.
Check the stock for cracking or splitting. A stock that is broken is one that you don't want to shoot. Replacement stocks can be pricey depending on make and model, you don't want to double your cost by replacing parts right away.
Know the cost of a new rifle in the same caliber from the same manufacturer, oftentimes a used rifle is priced way to high. A copy of the Blue Book of gun values is a handy reference.
Good luck shopping!
Jimro