How do I know if the torque convertor is locked?

Why not have a look at this forum, lots of Prado owners/users Aust 4WD Forum I'm sure that someone there would be able to give you a consice answer.
In general. if the transmission is in D D+ O/D or whatever it is on your vehicle, it is in overdrive, the torque converter (device in your auto transmission that apportions power/torque to the drive wheels and is itself using fluid to smoothly transfer power from the engine) will mechanically lock-up. This means that the transmission is not longer 'fluid coupled' but actually mechanically coupled. Designed to improve fuel economy and usually engages at a given speed possibly in the range of 80 - 90 km/ph. When you get to know the speed, in many cases you can actually hear/feel the torque converter lock-up and detect a slight drop in engine revs.
Many vehicle manufacturers advise against towing in Overdrive as it places additionl strain on the transmission and can cause premature failure. It is often found that towing in Drive (not Overdrive) results in little if any change in fuel consumption (obviously depending on speed, terrain, all-up weight etc) and the vehicle performs better. Others may see it differently.

When a torque converter lockes up it is mechanicaly connected to the engine.I have found that an easy way to test this is at a given speed say 90kph back off on the loud pedal a bit & watch your taco. if it drops dramaticly the converter is not locked if it stays about the same revs it is locked. The same goes for giveing more power, if the reves increase slightly it is locked if you get an increase in say 3-500 rpm without the corrisponding increase in speed you know that the converter is slipping. best to try this without a load & you will get to know what reves relate to what speed.Remember if yor converter is locked the reves will be the same towing 1 tonne or 100 tonne.

Thanks all for the kind help. I'll check out the Prado and the torque convertor on our next trip away in May.

Steve