That answer will vary based on the webinar provider. Given that you are asking on an (ISC)² centric forum, I will answer based on the official (ISC)² webinar provider, BrightTALK.
Certificates-of-attendance for the webinars you have recently attended can be found on BrighTALK under "Viewing history". I recommend downloading them as PDFs into an "evidence folder" on your PC because they disappear after a period of time (90 days???).