Ask a Mechanic.

Do you guys take one look at a woman who comes into a mechanic shop alone, determine by the way she looks whether or not she'll be easily scammed, and try to get her to replace shit that doesn't need to be replaced? Because it seems like you do.

My friend has this trick that I'm going to try sometime. Every time a mechanic recommends that replace a part on her car, she tells him to go ahead and do it, but that she needs the part back because her brother is a mechanic and he's out of town, but he wants to look at the part when he gets back to make sure it really needed replacing. She said she's done this a couple of times, and inevitably the guy will come back and say, "Good news! We were able to clean that air filter (or whatever) and it should hold you for a while."

Jam it back in, in the dark.