A short check before setting off saves you from most problems on the road and from a ruined day for the whole group.
Squeeze the lever and press the pedal: they must feel firm, not soft. Check the brake fluid level in the reservoir on the handlebar and the thickness of the pads.
Headlight, high beam, brake light on both brakes, indicators and number-plate light. Ask someone to look at the back while you apply the brakes.
Check that it's lubricated and has the slack given in the manual. A chain that's too tight or too loose wears quickly and can jump off the sprocket.
The oil level, in the sight glass or on the engine dipstick, and the coolant, if the motorcycle is liquid-cooled. Also look under the motorcycle for leaks.
The list is indicative. For exact values (pressures, chain slack, type of oil) use your motorcycle's manual.
Other guides: Motorcycle gear · First group ride · Hand signals