You'll need to be able to prove that you paid for diesel/petrol and either got contaminated fuel or the wrong fuel. Your receipt should clearly show that you bought the correct fuel. The next step would be to have the fuel examined and the contamination identified.

Proving the garage is guilty will be difficult without other complaints. For example, if you have run the tank low, it's possible the contaminate has been in the tank for some time and sat at the top of the fuel. This means the contamination may have come from somewhere else and the garage in question is entirely innocent.