I've always thought the easiest test of an alternator is to just disconnect the battery while the truck is running. If it keeps running, your alternator works. If it dies, it doesnt'. It's a start at least..
82 fj40, 99 uzj100, 06 gx470
97 DR650, 05 525EXC, 01 XT225, 89 TW200, 09 TTR125, 07 TTR90, 07 CRF50