There has been a rash of people showing up and making an offer to sample your fuel for contamination at no cost to you.
They pull a sample and place it in a bottle for you to see. It always shows water and debris contamination. The next step is they give you a proposal to clean the fuel.
Through slight of hand they spike the sample that is shown to you by putting the fuel in a bottle that already has water and debris. When someone samples your fuel make sure you watch the whole process and look at the containment jar ahead of time to make sure it is not spiked.
It is important to put the resulting sample in perspective. You are taking a small sample from the very bottom of the tank that might have a very small amount of contamination, contamination that has no effect on your fuel. To give a true picture of the state of your fuel you should use a paste to determine the amount of water in the tank and sample at several levels.
REMEMBER: A FREE TEST IS WORTH WHAT YOU PAID FOR IT
RECOMMENDED ASTM SAMPLING STANDARD