We do predive checks every dive.
We each do a modified valve drill (in water)- makes sure gas is on and valves can be reached.
bubble check for each other (in water)- makes sure long hose is deployable, backup is where it needs to be and both regs work
bubble checks (in water) - makes sure nothing is leaking.
Modified s-drill (unless a full s-drill is called for)-makes sure long hose is deployable, backup is where it needs to be and both regs are functioning...you want to make darned sure your bud can give you gas if you need it!
Check lights by turning each on and off (usually in water).
Head to toe equipment matching (usually in water)- makes certain every one knows what every one else is carrying and where and that you have what you need. Hopefully, with the exception of special equipment every one is carrying similar items in similar places and it only takes a few seconds.
Gas planning...check supply each diver calculating turn pressure (or whatever is being used)
The designated leader briefs the dive and every one briefs the dive in turn. Depending on the dive this can be very simple.
In fact for a basic planning check list I like saddddd as GUE teaches it. sequence, air, depth, direction, decompression, distance and whatever other d's you can think of. One addition might be any specific tasks that might be planned on the dive.
It looks like alot but once you have it down the whole thing only takes a few moments. Some of it varries a bit depending on where we're diving. For instance jumping off a boat into cold rough water we won't be spening much time farting around on the surface so some things get done before getting in or a few feet below the surface where things are a little calmer. On a shallow daytime OW dive we may not need to check three lights each. In a shallow quarry where we can't go deep and it doesn't matter where we surface decompression may not be a factor, rock bottom is 500 psi and the direction plan is pretty loose and distance can be just bouncing off quarry walls until we're tired of it.
We also do negative pressure checks on our regs before turning the gas on which is something that too many folks don't do. With the reg attached to the valve and the gas off just suck on the reg. You should get nothing. This makes sure that there are no leaks in the reg as through a torn or missing diaphragm (main or exhaust) or a cracked housing. This is something that breathing off the reg while out of the water won't tell you.