Well, if you're using BrightTALK then they have a history section in your profile.
Otherwise, browser history, or if you have them firewall / proxy logs might tell you.
NB - It's always recommended to keep a record of any CPE activities you undertake, especially if the CPEs are meant to be auto added so you can track if they are or not, or if you're not manually adding them straight away just in case you forget them.
As previously mentioned, we should all keep at least simple records of CPE activities, including date and at least one item that can be submitted as supporting attachment when reporting the event. I often use a screen shot of the opening slide, plus have the URL in my description.
As mentioned elsewhere BrightTalk webinars are submitted on your behalf automatically. Accepted CEUs are listed under 'View' in your personal dashboard.
I keep an Excel spreadsheet out of both determination and old habit to guard against audit. Since CEUs are automatically audited today not sure if their is any real need but I still keep my spreadsheet and follow up once or twice a week to be sure I am getting credits for my numerous ISC(2) certs and frankly, keep me on track.
Good question. Step away from the Excel spreadsheet. This should be automated!\
Btw. Major learning platforms like Safari Online do an extremely poor job (i.e., they don't tracking learning history) and that is something that I have requested as a feature numerous times...