To test the lockers you just want to see if they engage. Is best to do on gravel or similar, but if not it's no great worry - just try and drive in a straight line if on tarmac.
As far as I'm aware you simply cycle the F+R diff-lock switch round to engage them. The diff-lock lights on the dash will start flashing and will then go solid if they engage. You sometimes need to drive the truck forward a bit to get them to engage. The F+R diff locks will only engage if the centre diff is locked. This may happen automatically when you turn the F+R diff-lock switch but if not simply engage the centre diff-lock youself by either pressing the button on the dash (older trucks) or engaging low range first if there's no button (will then engage centre diff lock automatically.
If you're on gravel or grass or similar and the lights on the dash keep flashing (i.e., the diff-locks are not engaging) driving gently in tight circles can also sometimes help.
It is not unusual at all for at least one (usually the rear) to not engage if they haven't been looked after or used regularly. Sometimes sorted through a strip and clean, sometimes is an electrical issue, though may mean a new actuator.
Good luck.