Here you go: CHECK - Clothing Harness/Helmet Environment Connections Knots.
Clothing should be tucked in to the harness or out of the way so that it doesn't get stuck in the rappel device.
The harness should be correctly tightened - the belt is often doubled back on itself so check for that. The leg loops should be tightened as well.
The helmet chin strap should be buckled and the headband thing tightened on the head.
You can optionally add Hands and Hair to this as well. Hands means gloves should be worn. Hair should be tied back if it is long. The new acronym would be CHHHHECK.
If you're in Headless Hen or Hard Day Harvey, it would change to CHHHHHHECK or CHHHHHDHECK, respectively.
Verify that the weather is suitable for canyoneering if this is the first rappel. Check that there are no loose rocks or things that can come down on the belayer. If there are, make sure the belayer knows and that the rappeller is mindful of their steps.
All carabiners should be locked (click test). The rope should go properly through the rappel device.
The Click test is to just squeeze the gate of the carabiner. If it clicks, it is locked. If it does not click and opens, it needs to be locked.
Anchor should be EARNEST and knots should be checked before the first person rappels. Otherwise check that any knots are correctly tied. This includes any autoblock, Valdotain Tresse, or top-rope belay knots.
Another one I have heard with a similar goal is ABCD:
Airway Breathing ... just kidding, it's:
I first heard CHECK from the GSLCBSACIC (Great Salt Lake Council Boy Scouts of America Climbing Instructor Course).
I can't remember where I first heard the ABCDs but if you want credit for it tell me you invented it and I'll tell everyone you were the genius behind it.