Asking them for the dollar amount to fill is a good idea, but it doesn't keep them from trying the bill switch that we are talking about. Doesn't matter what you ask them for, when you hand them the right bill, it magically becomes the wrong bill. 500 peso notes magically turn into 50 peso notes, 200 peso notes magically turn into 20 peso notes. It the most amazing thing, who knew David Blaine worked at a gas station? They do the switch light lightening and then they give you a big smile and say "Oh senior, it's 200 pesos, you gave me 20 pesos and hand you the bill back." Many tourists simply chock it up to themselves being less than adept with the foreign currency and figure they did make the mistake and give them another 200 peso bill.
I forgot to mention that you should be sure of what you gave them (as DandyDon suggested), and if they pull something like that, just say "no, I gave you 200" (or something more harsh) and drive away. If you have time, it isn't a bad idea to go inside and complain.
---------- Post added July 9th, 2015 at 07:59 AM ----------
Like Don is saying They are like any criminal, they size up their targets, when you create a presence that shows you're on point they typically look for an easier target.
Basically what I do is pull up and get out of the car, tell the attendant what I want, stand there while he pumps, look at the pump to read the amount and hand him the amount required. When handing it to him for instance a 500 peso note, I make eye contact hold the bill up and show it to him, sometimes I snap it, but always say the amount out loud, hesitating for a split second, the attendant typically makes some sort of acknowledgement with a head nod or something during this tiny awkward moment, (he's probably thinking WTF gringo????) then I hand him the note and it's all good. It's one of those silent "I know, that you know, that I know that you know that I know what you're up to."
They generally give up trying to rip you off when you get out of the car, stand there and make eye contact and get acknowledgement. You give them the impression you know what's going on and they pass on you and will wait for an easier rube they can take advantage of. Never had a problem since doing it this way.
The problem with handing them 500 (unless of course the total is 500) is that they now have the money and owe you money. They will try every which way to short change you and pretend not to understand what you are saying and you lose money if you just drive away.