Learn How to Test Ride a Bike
The main things you're interested in on a test ride are to find out how the bike handles bumps, to find out how the brakes and shifters work, and to see if it was well-built. Before you go to the bike shop you should have a plan for where you intend to take the bikes you test.
Take the bike on Hearst or somewhere similar; how much does it jar you when you're going downhill? When it hits bumps, do you hear unexplained rattles? You should expect your rear derailleur to snap when you hit bumps, but if other things on the bike are rattling it's a sign that something is cheap or poorly fitting.
To test the shifters, start by riding on a flat road and shifting through all the gears. Does it easily shift into the lowest gear? If you over-shift into the lowest gear, does the derailleur sound like it wants to self-destruct into the spokes? (If it does, stop over-shifting, but that's a count against the bike.) Does it shift cleanly into the highest gear, without jumping over the end?
Can you shift between the front chainrings easily, without the chain jumping off? On a triple crankset it can be expected that the chain will jump off sometimes when shifting to the smallest chainring, but on a double it should never hop off and it should never hop off when shifting to the large chainring. Try out each cog for a while; does the chain run smoothly and quietly on each cog, or does it chatter or jump off one or more?
Try to downshift while pedaling uphill; it's harder to shift with tension on the chain and cheaper shifting systems will fail to. If you're spending$500 or more, though, you should insist that the bike be able to make this shift cleanly.
When you're riding uphill, do you hear pinging sounds in the wheels? If you do, they were poorly built; the sounds will eventually go away but the wheel will need to be re-trued.
Head downhill and hit the brakes; do you stop smoothly? Do you feel like you have control over your deceleration? Cantilever brakes are somewhat rougher than side pulls and feel spongier, so expect that.
Walk the bike while turning the handlebars, do they turn smoothly through their range of motion, or does it feel like there are notches at various points (especially straight ahead)? Try some medium-speed turns on pavement; does the bike corner solidly (if a road bike; knobby-tire bikes corner horribly on pavement)?
If you are planning on riding off-road, find a dirt trail or two (there are some on campus) and see how the bike handles them; can you accelerate from low speed on the dirt? Can you turn without skidding? Can you shift on a bumpy section?
As I said, try out a number of different bikes in your price range; there's no way to measure how good a bike feels to you. And most importantly, once you buy it, RIDE BIKE!