And if what you say is true, it'll be even easier to tell. It would be absolutely black the day after an alleged oil change which is surely improper.
Originally Posted by quality_sound
After about 500 miles that "new" oil will look just like oil that's been in there for 5 or 6 thousand miles. This would be a good check to do at the dealership though, assuming you still distrust them enough to feel the need to check.