You want to check the condition of:

All belts and hoses. If you see any cracks, it isn’t a big deal, but knock 20 or 30 bucks off the price for every belt or hose that needs immediate replacement. (except vaccum lines…the really little ones some cars have…those are cheap and easy)

Look for oil leaks or coolant or transmission fluid or power steering fluid (usually the same thing as transmission fluid, just in a different place) leaks. They’re easy to spot if the engine is covered in dirt…anyplace shiny has some kind of a leak. If the engine has been cleaned, though, you have to be on your guard. A dirty engine is a GOOD thing.

You want to listen to the engine (and the rest of the car) very carefully while you are test driving. If you hear ANYTHING out of the ordinary, and they don’t have a damned good explaination, walk. You’re particularly interested in metallic squeaking sounds that don’t sound like they’re inside the passenger compartment (that’d be suspension trouble), and any sort of whining (belts) knocking, or pinging from the engine…or even just sounding too “rough.” It should, to use a cliche, purr like a kitten.

After you test drive, you want to stick your head under the hood and smell the engine. If you smell anything other than a faint oily scent, walk. Either that or they’d better have a damned good explaination, and account for that problem in the price.

You don’t need to shake or jar anything. Maybe test the play in the belts a bit…they shouldn’t move more than 1/2 inch. That’s only a big deal if the belts are on an automatic tensioner…you’ll know it if you see it. It will be a pulley that doesn’t look like it’s doing anything, that’s on an arm that looks like it can swing back and forth. Tensioners can be pricey on some cars.

Oh, and you want to check the condition of the fluids. If the transmission fluid (auto) is dark brown, you probably don’t want it. If the oil is REALLY dark brown and opaque, you probably don’t want it. If the coolant is brown and murky, you probably don’t want it. If there is foam under the oil fill cap, or on the dip stick, or if you can see oil in the coolant (I have actually seen that), you REALLY, REALLY do not want it.

Oh, and about leaks…if it’s a really slow leak, that rarely if ever has drops hit the ground, it isn’t a terribly big deal, particularly if it’s just from the oil pan gasket or the valve cover gasket.

Finally, once you’ve narrowed down your list (you do have a list, right?) to the few cars you’re actually going to test drive, you should look up each model and find out about any common problems peculiar to that model. Autos.msn.com and Carsurvey.org are good sites for that.

One of the most important things to check is the oil pressure to tell engine life left.

The most important thing is to take it to a reliable mechanic and let him check it out. It is well worth the $$$$$$$$$$$$$.