you can call the DVLA to confirm the date your test was passed if you are struggling, but I think it is on the photocard licence (if you have one). You should also find it on the paper licence
If you have the pink plastic bit of your driving licence and look at the categories on the back mine shows the date I passed my test on categories B1 all the way down the card
I don't know if a NI licence is different so my answer is based on my knowledge of ours.
The old paper ones don't show a date so DVLA is the only way ahead. If you have a photo card then the date of passing your test is, I am told, on that.
There is a date on my card under vehicle types but I`m not sure if that is the date I passed my test or the date that my first licence ran from - there was probably an interval between the two.