we had a pen at the shop where I worked and were to test any bill 20 or larger-by marking it - and if it turned a certain color it was fake - I never got one. Also , the gas station is one place where scammers do function so watch out for them @angrytwin
Yeah, they have to have electronic machines everywhere here and they test a lot. I hear the pens aren't 100% accurate - but I have never seen one.