I've read the T&C for that fairly closely, I think its actually administered by Visa rather than CNB. I doubt they will affirmatively contact people about eligibility. One way you could potentially track that is to look at your monthly base point earnings on the reward site. That should line up closely with the annual spend. However if one is right on the cusp, I am not sure if its monthly statements summed up that count, or actual charges that posted in the calendar year, or actual charges on transaction date etc. Bottom line if you were really close to that 50k, I'd just go over it.
Which lounge would you pick if you got the benefit?