Good used bikes are generally pretty easy to spot, they have the brand names and features that are also on the bikes currently available at the better bike stores.
Thus the first step is to go to some place like Kozy's or Village Cycle Center and check out a nice Trek, Cannondale, Specialize, Gary Fisher, Giant et cetera. Note the kind of frame material (aluminum or CrMo) the components (both the BRAND and the sub-line/quality), as well as features (this includes everything from the brakes to the size/type of wheels, suspension, rack mounting points). Then go shop the used the stuff.
You want to compare apples to apples, but you have to have some smarts. Although the visual difference between a true off road mountain bike and a more citified commuter/crossover might be minimal the details are what matters. If you prefer speed / easy peddling you might want 700C wheel, if you want a tougher wheel / more offroad choices go with 26" hoops.
If you are worried about hills you want a bike with triple chainrings for a wider range of gearing.
If you find a bike that is CLEAN you are way ahead, as dirt can literally hide damage from crashes that may render the bike weakened / unsafe. Rust is also a tricky thing, as literally any of it in a critical area might be lead to an unsafe condition -- the joints where the frame tubes meet can have as much stress as an aircraft weld! Evidence of maintainence of any kind is a good thing in a used bike, and well adjusted brakes, derailuers, and chain will make for a safer more comfortable experience.
It is stupid to buy a "Target quality" bike for an adult new or used. The frame will be heavy and not hold up as well as bike store quality bicycle. Some shops will simply not work on the low end components as they know that know that no matter how much shop time that they can bill to try making that stuff work it will not result in a happy customer.
If you don't find what you want on Craig's list you can generally get a good deal at a police auction, and when the stuff is "fresh" it may only need a minimal visit to a bike shop to get into tip-top condition.
Chicago Police
(btw back in college one of my many jobs was as a bike shop mechanic and back then we were just starting to see the birth of "Mtn Bikes" -- I think the stuff that sell now is stronger and lighter than the stuff that the "pros" used. Times change!)
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Help on buying bicycles
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