To check for fuel contamination before filling up, pump a quart of fuel into the bucket, then wait a minute for settling to occur. Any water or sediment should be clearly visible against the pail's white bottom. Good-quality diesel fuel (40 cetane or higher is preferable, and it's often required by engine manufacturers) should, if it's destined for road use, look like honey; off-road/marine diesel is dyed red, so it should look like diluted cranberry juice. If your diesel looks like coffee, pass on it. You should certainly be able to see through a quart's worth to the bottom of a gallon container. If your diesel passes the bucket test, you may want to fuel up without any further delay, especially if you regularly purchase fuel from the vendor. However, if the sample is suspect, try pumping the fuel through a filter funnel. This will remove the coarsest sediment, but don't expect miracles. Large amounts of water or ultrafine dirt won't be stopped by this rudimentary filter system. Forget using one if you're in a hurry-they're slow to drain.