I usually have a good look at the driver's seat first. Are the springs noisy and sagging? Do all of the power seat controls work? On a 49,000 mile car they should all be in excellent condition. Another is the speedometer. If it is jumping around, like mine does, it could be a sign of the cluster being pulled and a different speedo installed or tampering. The cable is fragile and will get damaged this way. The spark plugs should show no signs of oil fouling. Parts under the hood should be original to the car. I just bought a 1988 300E with 89,000 miles on it (so they say) and I am finding all kinds of high-milage clues. I knew it was a shady deal though because of the driver's seat and speedo, but it didn't matter because it runs like a top. Yesterday I saw that the driveshaft has red, yellow, and green painted stripes on it and I don't think that it came that way from the factory! Also my flex discs are shot and the car clunks into gear when cold.
Chris