The wear patterns on the brake pedal cover are usually a giveaway. Under typical driving conditions, it usually starts showing some wear at about 100K, and significant wear at 150K. Most people don't bother to replace them, and certainly not before 150K.

I recently looked at a 240 with allegedly 76,000 miles. I was suspicous at some minor clunkyness in the rear end. Excessive brake pedal cover wear confirmed my suspicions.

Even if the brake pedal cover appears to be new (and perhaps replaced), such that nothing can be deduced from its condition, you can take the pedal cover off to look at the pedal itself. At about 150K the original pedal cover wears all the way through, and then the sole of the driver's shoe rubs the black paint off the pedal, leaving shiny bright metal. So take the pedal cover off and have a look. No car with fewer than 150K miles should ever have shiny metal exposed on the lower right corner of the brake pedal.