Disco 4 (LR4) Diff lock

I have ax2926 disco 4. As an approved used I went on a landrover expense day. THE man there was explaining the diff lock for me and the symbol.

I used the disco today to launch a heavy boat on a beach which included going over soft sand. I had the 4x4i on to monitor the diff lock symbol. At Jo time did it show as locked or any of the thee sections in the ring change. I was anticipating that with 5 adults and a 17 foot speedboat that the lock would be needed. I am now wondering if it is working.

Is there a way to test it.

Only works in certain modes ie rock mode/ low range?
Might say in handbook?
Centre diff lock is std fit
Rear diff lock expensive option
Not heard of a front locking diff