Buy a new car - don't buy someone else's lemon, new-car salesmen have advised potential customers for years.

But as new-car prices continue to climb and the cost of gasoline - if you can get it - consumes and increasing amount of the average family's transportation budget, buying a used rather than a new car will become economically appealling, and necessary.

"Buying a well-maintained used car is an attractive, inflation-fighting alternative to a new car," claims former presidential consumer adviser Virginia Knauer. "You can save 20 percent on a 1-year-old car and 40 to 50 percent on a 2-year-old car."

Furthermore, she says, the old saw about buying someone else's lemon need not be true.

"You don't have to be an automotive genius to avoid buying a lemon," she argues. Knauer is now a consultant to Hertz, which in addition to being the world's largest rental car company is also the largest seller of used cars (those it used to rent).

Beware of cars that are being sold "as is," and search for a used car that carries some sort of warranty, she says. Never buy a used car without test-driving it extensively over all sorts of terrain and in all sorts of driving conditions - highway, start-stop rush-hour traffic and the like.

Ideally, the consumer should not buy a car that has not been looked at by a trusted mechanic. (But unless one has a friend who is a mechanic, the cost savings will evaporate if a potential buyer drags a mechanic to 10 or 12 lots following Knauer's advice to shop around.)

The average consumer, by following some simple procedures, can weed out a lot of cars on his or her own.

The other night, in a demonstration at the Waldorf sponsored by Hertz, Knauer gave these pointers as she walked reporters through a standard, thorough examination of a used car:

Check the exterior for signs of rust and signs that the car has had rust problems. by running a hand over the paint job, one can feel the small indentations in the surface that indicate the metal has been sanded down and repainted, a sign that the car has had rust problems. A car that rusted once probably will rust again.

Look at the chrome. If the chrome is dented heavily, the car likely has had hard usage.

Open the doors and look for evidence of a massive repainting of the car. If the color of the interior door frame, or a part of it, is different from the exterior, the car has been repainted. That usually means the car has had massive body damage in a serious accident.

Look at the interior of the car for general cleanliness, but also look up under the dashboard and in the glove compartment for signs of flooding - usually a well-defined line, above which the car was dry and below which it was under water. Many cars on wholesale lots were flooded this spring and the electrical system of a car that was flooded will always be trouble. (A shady used-car dealer usually cleans up visible signs of water damage but often forgets the glove compartment and under the dash.)

Check the odometer, then look at the brake and the gas pedals. If the rubber casing on the pedals is worn almost to the metal and the odometer reads 40,000 miles, be suspicious. It is a federal offense to roll back odometers, but some folks still try it.

Make sure all the accessories work. Check the lights - interior and beams - and the turn signals. Turn on the radio, the air-conditioning and any gadgets the car has. Make sure the windows roll up and down, the vents open, the heater heats and the defroster defrosts.

Lift the hood and inspect the engine for general cleanliness. Tug at the wires and make sure they are not frayed. Look for leaks from any of the hoses and be sure that no hoses are sealed temporarily with tape.

Loosen the insulation on the underside of the hood. If the engine has had a fire, the paint on the underside of the hood often has telltale blisters.

Pull out the dipstick and make sure there are no little bubbles of water in the oil, a signal that a gasket might be blown somewhere. Pull the dipstick for the transmission fluid and make sure it is bright red. Smell it too. If it smells like varnish or has a burned smell it probably means the transmission needs an overhaul.

If the radiator is new, be suspicious. It probably means the car had a front-end collision. Look at the cool-ant, make sure it is clean and free of oil.

Press down with all your weight on each of the four fenders, then release. If the car bounces more than once, it means the shock absorbers are going - or gone.

Check tire treads. If the wear is uneven, the front end of the car is probably out of kilter. Put a Lincoln penny into the space between the treads. The penny should go in as far as Lincoln's head. If it doesn't, the car will need new tires soon.

Thoroughly wet down all four tires with a hose, then drive the car slowly, in a straight line, over a dry patch of concrete. There should be only two parallel tracks. If there are four, it means either an alignment problem or a bent frame.

Open the trunk and check the spare. Make sure it holds air. Also look in the trunk for signs of water. Look for signs of major new welds, an indication the car was in an accident.

Drive the car over some clean newspapers or a clean concrete area and let it idle for five minutes. Then check the paper or concrete for signs of leaks. Black dots mean an oil leak. Reddish dots mean a transmission fluid leak and brown dots means a gas leak.

Check the exhaust to be sure that it is clean.

When driving the car, make sure the brakes are firm and smooth. Test the steering. The wheel should not move more than a few inches before the car starts to turn. CAPTION: Picture, no caption