On my 80 it used to automatically lock the center diff when low range was engaged, however that was bypassed, as I have a button on the dash for the center diff lock, so it can be locked in high range originally, now I can chose when I want the center diff locked, and a rotary dial to engage the axle diff locks.
You should have a amber diff lock warning lamp in the instrument cluster to warn you when the center diff lock is engaged and a pair of red ones for the axle I believe, if diff locks are fitted.
Should illuminate on ignition on as a kind of lamp test, mine does anyway.
Did Toyota do away with diff locks like Land Rover did when they started to use the traction control instead of mechanical diff locks?
Try checking for information tags on the diff housing as StarCruiser said, also you can try jacking her up off the ground and selecting low range and seeing what happens when you spin a wheel by hand, if the enter diff is locked then a wheel on the opposite axle should turn at the same time.