nikon flash, sb 28dx. put in fresh batt,get hold of the corresponding instruction booklet,mount it on a body with lens, fire some blank shots to check for:
1. recycling time (shld be around the same as stated in the manual)
2.zoom range detection (use lenses from 24mm to 100mm)
3. does the light look "yellowish"=> old flash or "bluish"=>not old, quite new
4.for the head:tilt, pan, the pullout mini-reflector+bounce card
5.physical:hot shoe contacts(rusty?loose?),batt compartment(any leaks?).etc
6.does selecting the functions on the flash itself will the results be indicated on the cam body's LCD panel?
7. go thru the slow sync, rear sync with a real slow shutter speed(ie flash should fire just b4 exposure ends), front sync, default x-sync, "default speed in A-priority mode sync", continuous shooting eg 5fps sync..
8.anymore?..offhand i cant recall..
some functions of the sb28 series may not be accessible with the lesser nikon bodies..duoble check carefully on both the cam body n flash manuals