Checking your bike set up is an useful discipline before a ride. It saves you stopping on the trails and holding the group up plus your more likely to have the tools at home than out with you. In essence it equates to more time in the saddle. 

I'd be lying if I said I checked my bike each time but this is a simple guide especially before a big day out riding. 

• check your back and front tyres levers are on tight (not too much you cant get them off with wrist strength alone). I recommend that the lever is  face up so if knocked on a rock they don't come loose. Test by rolling your wheel and this will let you know they are not messing with your brakes. 
• take a cloth and wipe your chain down. Running the chain backwards through the cloth does this well. This removes all the dirt and old grime. Then re lubricate. Unless a long ride in the wet I recommend you use dry lube as this maintains your chain better.
• Check the pressure in your tyres (35 40psi for tubes) 28-38psi for tubeless.
• Check every bolt
• Check your pedals spin smooth.
• Give a little ride and check you gears shift
• Check your brake pads aren't warn.