Hi Chris, is your 100 a UK model as AFAIK all UK 100's had RDL's. I've only recently got a 100 and am still learning about them but I recently tried an Aussie spec GXL 04 100 petrol and that had an LSD rear axle but bought an 03 Aussie spec Diesel in the end with a RDL. The RDL is so corroded it is split in half so I know at the moment which would work best. If you have an LSD axle it will always work and you will never have "one wheel drive" so to speak.
If it is LSD then it should have a sticker on the rear of the axle saying "LSD oil only" but if you jack both wheels (rear) up with the transfer box in neutral and turn one, the other should turn the same way, if you put one wheel back on the floor and try to turn the wheel it should not turn if the LSD is working.
Hope this helps.
Andy