Find a tree on the field. The dark backround will contrast the white BB.
Do not look through the optic. Shoulder the weapon and aim at the tree but raise your head when shooting and loop ever the sights.
If you have a bright light you can use that in dim light to make the shots seem to light up. Or you can go out in the dark with a tracer unit and glow BBs and become familiar with the way BBs fly.
Airsoft, where nothing is hurt but feelings.