there are 5 things i check on my camera before i begin every shoot:
make sure it's on a setting that works for the light situation you are in- sometimes i've done a shoot at night and have forgotten to change the setting from 1600 ISO to 200 ISO. this helps avoid the grain that comes with higher ISO. if you notice you're having to expose with shutter speed and aperture and the numbers seem weird, look at your iso. you've probably shot a whole shoot in daylight at 1600 and yes, it's happened to me :(
2. battery life
always check your battery juice, it's such a shame to have your battery keel over in the middle of a shoot. also, make sure you always have a backup just in case!
3. memory card
is there one IN my camera? i occasionally forget to replace them after downloading images to my computer. best to check before leaving for a shoot. also, i frequently format my memory card before a shoot.
4. exposure settings
this is especially important when i shoot kids, since it's best to have it all set before they run around- they don't particularly care if you're under or over exposing and pretty much don't don't wait for you. kids are simple and energetic folks and you have to prepare your camera for that.
is your camera set to manual or automatic focus? i go back and forth depending on the type of shoot i'm conducting. it's important for me to check before the action starts happening.
* there are some other things i may look at such as white balance although i rarely use it, clean the lens if smudged or otherwise unpresentable, and make sure i have backups of things like memory cards, batteries, batteries for my external flash, Pin It Now!