Was my emptying done properly?
Many garages use the so-called « suction » drain, which involves putting a pump into the hole where the oil is poured, in order to suck in the used oil without undoing the drain plug located under the vehicle. Faster and easier to implement, this solution is not satisfactory, as the dirt and residual sludge remain at the bottom of the tank and will quickly stain the new oil. And all mechanics know that.
To verify that your emptying has been done correctly, before dropping your vehicle, make a chalk mark on the drain plug under the car. If your oil change includes replacing the oil filter, do the same on the filter. When you get your vehicle back, make sure your markers are gone. Indeed, if the cap has really been disassembled, the drain key, the oil flow and the hands of the professional on it will have erased the chalk. The absence of a marker on the filter will prove that it has been replaced.
Monitor the progress of operations
Do you have a little time? Use it to observe and follow the repair. A mechanic who has nothing to be ashamed of will not forbid you to see what happens in the workshop. Moreover, many car centres offer waiting rooms equipped with large windows overlooking the workshop.
If you are categorically refused to see or be present at the repair, it is suspicious.