You don't need to depend on luck to find a good used car. Even a mechanical novice can boost his or her chances of separating clunkers from gems on a used-car lot with a systematic examination, from the body panels to the tires to the condition of motor oil and transmission fluid. A layman's inspection and test-drive don't substitute for having a professional mechanic look over a car. But an informed check of the basics on a car can weed out some severely troubled cars and increase a shopper's odds of picking a good car to take to a mechanic. What do you look for in a good used car? Dick Fox, the third-generation proprietor of Fox and Fox, 512 Van Buren St., took a few minutes recently to offer some advice on spotting a good find among used vehicles.