log in to your ibanking.
Click credit card
Today points minus bgn points = points earned for the month
Correct me if I'm wrong, but I don't think it's possible to get the beginning points balance unless you login to the site the day before and make a note of the number?
Then, you also have to take away the points that are given from your normal transactions that have been settled (appearing in the unbilled section on the same day as the bonus points day). Also take note of any other DBS cards, as spendings on those cards will also add points to this total number.
After which, obtain the transaction list (I'm lazy, I just copy & paste into Excel). If everything adds up, great. Otherwise, you'll have to do a trail and error to see which transactions didn't earn the bonus points.