I'm looking to a used amp in the next few weeks, but I'm not sure how I should test it when I first see it to make sureit isn't damaged in any way and will last a while.
The Setup:
Fender Standard Stratocaster
Crate GFX-30
Vox AD50VT
That's the risk with buying used. What if it doesn't work?
hopefully the buyer will havea return policy.
Buying used results in spending less money allowing for a better quality item. Of course new; you spend usually twice as much, but have peace of mind knowing you could return/repair if you encounter a problem.

That said, buying used is best done in person and testing the item you are considering purchasing. Of course look for cosmetic problems (especially the kind that may have damaged the product's operating ability) - such as a large dent from a fall.

Test all features to make sure they all work. Listen for crackling in adjustment pots. Of course look for burnt areas, or smoke (actually more of an electrical type - sorta like burnt plastic) smells. Spend as much time as possible with product running to make sure there is not an intermitant issue.

Following basic inspection and asking the seller questions (such as why it is being sold) results in getting quite the deal! I've bought and sold alot of stuff and am still batting *100 success rate! ( knock on wood! )