because i'm nice, and bored, i googled the test procedure. source:miesk5/Gacknar
If the locking hubs are the automatic type, they engage by themselves; if manual hubs, each hub on both front wheels need to to be turned to Locked before pressing any buttons on dash.
Manual Hubs & 1356 Electric Shift Operational Test; "... (engine off, hubs disengaged), crawl under the truck with the transfer case in 2HI, you should be able to grab the front drive shaft and turn it by hand. Also with the hubs disengaged you should see the front axles turning freely. Now engage the hubs and you should not be able to turn the drive shaft by hand. Jack the front wheels off the ground with the hubs engaged, turn the drive shaft by hand, the wheels should turn. The 4x4 light should not be on in 2HI, when in 4LOW you may have a low range light on, (not all trucks have that light) at least mine doesn't..."
Source: by Seabronc (Rosie, Fred W) at Ford Bronco Zone Forums
Auto Hub Operational Test; Remember to block the wheels. This would work for testing manual hubs too, just lock them 1st; "...Place transfercase in 2HI or Neutrall. 2. Lying under vehicle, turn front drive shaft (one direction only). 3. Automatic hubs should lock after 1 to 5 turns of driveshaft (one direction only). After step 3 the hubs should lock, youll know they are locked becouse you will hear a click from both hubs and you will no longer be able to turn the front drive shaft (in the same direction). If this is not the case then continue to step 4. If you can no longer turn the drive shaft (in the same direction) then skip to step 7. 4. If you can still turn the drive shaft (one direction only) then one or more of your Auto hubs is not working. 5. Continuing to turn the Drive shaft (one direction only). 6. Look at the u-joints for the front stub axles, if both are turning then both front hubs are malfunctioning, if only one is turning then the one that is not turning is locked and the one turning is malfuctioning. Replace as necosary (preferably with manual hubs, Auto hubs have Yams in them) 7. To unlock the hubs, turn drive shaft in opisat direction untill you hear a click from the hubs. To check for transfercase engagement. 1. Place pushbutton transfercase in either 4 HI or 4LO. 2. lie under vehicle. 3. You should not be able to turn front drive-shaft in either direction. 4. If you can turn the front drive shaft then your transfer case is not shifting properly..."
Source: by Gacknar