car check list?

I don't have any official checklists, common sense suggests though that you should document as much wrong with the car as possible.  Not to be an donut hole but you want to make damn sure that you don't get blamed--and forced to pay for--something you didn't love up.  I've heard stories from people who do this for a living and customers will want to blame you for everything from loose trim panels to leaky head gaskets.

I'd imagine you'd want to test everything electrical, turn signals, headlights, high beams, fog lights, daytime running lights, hazard flashers, power doorlocks, power windows, rear defroster, windshield wipers & washer, dome light, power seats, glove box and ashtray lights, and of course the radio, speakers, & antenna.  Note stains in the seats or carpet, any scratches, defects, or loose interior trim, and any funny smells.

Take a look at the car itelf too, note peeling paint, broken headlight/tailight covers, dents, scratches, broken plastic trim/grills, etc...

Pop the hood, take a look, note any leaks, noises, or anything suspicious.  Make sure the A/C and heater works, too.

Make sure the trunk opens too and that there isn't any leaks and note if it's dirty or not.

I know that might sound like an exaustive list, but IMHO you can't document enough before beginning work, otherwise the customer will want to come back after you for anything that's wrong with the car.  It's always worth covering your ass.  With some of the things I've mentioned logic suggests that you aren't the cause of the problem, but you never know what a customer might has up his sleeve.