I did say I'd help out - I just didn't think they'd take me up on it - at the last minute.

Okay.. It's a music video with professional grip, gaffer & DP but everyone else is at various levels of 'learner' (that includes me)

What sort of things should I be checking for (apart from the handbrake being off before I drive rapidly away)

I'm firing questions at them to get an idea of the schedule - we have a rough shot list but I'm wondering about basics - food, toilet facilities, shade, etc, etc

any hints?

I'd say get one copy of everything that's going to be printed to paper. Everything.

Phone numbers, lists, script, times, emergency numbers.. as first AD, you're running the place, so assume you will be. Know what everyone knows if you can, and then some.