If you do that you need to have a control sample for comparison. And various viscositys. I bet the differences will be too small to see with your eye.
Maybe you should get a different shop if you think the current one is so dishonest that you're willing to do all this extra work on a very slight possibility of cheating. And if you do find something, then what? Show the shop manager a cup of frozen oil? Find another shop you won't trust?
I think others are right that the chances of this happening are very very small. Why are you so worried? Does this kind of cheating sound like a good idea to you?