According to this article from Edmunds.com, the sixth step to test drive a used car is to be consistent.

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  1. 6. Be consistent.

    Despite advances in computers, the best vehicle-test computer lies between a trained driver's ears. For your used-car test, you are the test instrument. Establish and practice your routine using your current vehicle or one of the rental-car benchmarks. It's best if you can drive the same roads, but running the same speeds is crucial: 35 mph on surface streets and 60 on freeways will give good results without frightening the owner/salesman. 
    Make identical motions with the steering wheel. Here's a good one — while driving in a straight line at 35, turn the wheel smoothly from 15 degrees to the right to 15 degrees to the left and back a couple of times. Look for dead spots in the steering or notchiness that might indicate a problem with the steering rack or suspension bushings. If light freeway traffic grants the opportunity, at 60 mph do a series ofgentle sinusoidal turns — imagine unhurried lane changes — steering from 30 degrees to the left to 30 degrees to the right. (Warn the owner/salesman first!) Look for uneven response that would indicate worn shocks or bushings or mismatched tires. Go out of your way to hit bumps and notice the sounds and reactions of the suspension. If you see a problem that doesn't crop up anywhere else, re-run that route in your current car or the benchmark.