Very simple. next time you suspect people doing funny things, just don't sign the receipt with your real signature la, since they don't check anyways. Then next day call up your bank (Citibank is the best) and ask them to raise a dispute against the charge for you. Citibank will debit the full amount back into your credit card account within 24 hours. After this, they will continue to do their investigation on the Merchant(petrol station) the investigation will take several weeks but you don't have anything to worry about since they don't have your actual signature.
In any case even IF they manage to proove that you REALLY did make the payment, they will just charge your card the actual amount once again and add a RM20 administrative charge for raising a fake dispute.(again this applies to Citibank only).
I always raise disputes especially with funny merchants which double charge my card or worse still I even have 1 incident were the charge only appeared 2 years after the actual transaction date, I just complained and got my money back.