You should always take a sample from the hose nozzle before allowing fuel to be added to the aircraft. The plastic sampler that you use for the preflight water check is fine for this.
The main reason for this sample is a final colour check to make sure that you are getting the correct fuel, any contamination should also be obvious.
There have been several fatal accidents due to turbine fuel being supplied to piston aircraft. The engine typically fails at around 50 feet as the bad fuel works its way through the pipes, gascolator etc and reaches the carb.