How do I know if I am using Photo Stream or not ?

If you see a My Photo Stream album in the Photos app, and the My Photo Stream option is turned on in Photos ➜ Settings ➜ iCloud then you would be using My Photo Stream.

If the My Photo Stream option is turned off, or not visible at all, then you are not using the Photo Stream.

If you received an email its because at least one device you own that is logged in with your Apple ID has the feature turned on. Otherwise Apple does not send the email out to warn you.

Check all your devices. Though if using iCloud Photo Library too, photos are safely stored in iCloud regardless.

And if it's only one device that is using it, it's pointless as only its photos would be getting uploaded to the stream, but would still remain on the device that has it turned on when the service goes away anyway.

Perhaps this article from Apple may shed some insight and how and where to check

My Photo Stream - Apple Support (CA)

Good work there 👍

You say it more clearly than the Apple Support docs does