The most obvious and common test would be rubbing your fingers on a part, but it is not very accurate. Also, I see a lot of times in the industry utilizing a white towel test. Actually, white patch tests are sold, and these white patches are for seeing this visibly. However, it doesn’t give you a good quantitative number. To do that, if you have the equipment and a lab, you can weigh the part and then clean the part thoroughly. Then, weigh again to see how much the weight of the carbon is that was on the part to begin with.