You've just moved to a new city or town, and your car needs an oil change. Since you just moved, you also need to find a new mechanic for your car.
You look online or in the yellow pages, and you identify a mechanic. You drive over, and ask the mechanic how much an oil change for your car would be. You also ask if the mechanic has done oil changes before, and if you can talk to anyone who's had their car serviced by this mechanic.
The mechanic says they've changed the oil on hundreds of cars just like yours. There's a waiting room full of people having their oil changed, and you are welcome to talk to any of them, or you can talk to a few people here on this list. The mechanic has various certifications hanging on the wall for all kinds of oil change training he's received.