i've never received an email to switch email to yahoo

... How can I find out if I did overlook it?

It's not clear if Comcast/Xfinity is sending any out. If you log in to https://xfinityconnect.email.comcast.net/emailMigration it will tell you if the account you logged in with is ready to be migrated. If it is, that page will say:

You're invited to upgrade your email account to Yahoo Mail ... 

If the ID has already been migrated, the page will say:

Your Comcast.net email account is accessible at Yahoo Mail ...

If your email is not ready to be migrated and the page does not say "You're invited to upgrade ...", I'd imagine they just haven't got around to you yet. Or something is badly broken . . .

I don't recommend migrating. Comcast/Xfinity has recently changed their stance from "required" to "optional, for now". A number of posters who migrated have said they regretted doing so. 

Information on the Yahoo migration can be found at https://www.xfinity.com/support/articles/yahoo-email-migration-overview, https://help.yahoo.com/kb/new-yahoo-mail/SLN36803.html. and https://www.xfinity.com/support/articles/yahoo-email-migration-safety.