The way I found out was to go to Credit Karma, look under "Credit Card Use" and under each account name it lists the last time it was reported to the credit bureau. Some cards report on the statement date and some report on the last day of the month. It should give you a good idea.
For Capital One, look at one of your statements in the upper right corner and you'll see the billing cycle dates. Mine seems to be the same day each month. April 12 statement closing, previous months pay due was April 9.
Discover -> Activity & Payments -> Activity &Statements
Right hand box at the bottom is your statement closing date
The closing dates corresponded to what was on CK for me.
This is one of the many strengths of Credit Karma. Unlike many third-party suppliers of reports, it provides the precise date (month, day, year) that an account was reported to that bureau. Other tools often provide only the month/year. Consumers can then compare those dates with the exact statement date provided by the CC issuer.
Karma obviously has weaknesses too. One of these days I will try to do a write-up that explains what Karma is especially good for and what to avoid using it for. E.g....
Great for the reports themselves, great for Date Reported, etc.
If you have no closed accounts or AU accounts....
Good for age-related scoring factors
BAD tool otherwise for Age
If you have no closed accounts with balances or AU accounts....
Good for utilization
BAD tool otherwise for utilization
Also bad if you are close to a known breakpoint, i.e....
0.00%, 8.99%, 28.99%, 48.99%, etc.
BAD front-end tool for assessing derogs
Good tool if you really want to know your Vantage scores
Good tool if you want a free method of assessing your CBIS
(aka credit-based insurance score)
BAD tool if you don't grasp how almost no lenders or CC issuers
use Vanage scores or the degree to which your FICO scores
could be vastly different from Vantage