Spotify always sends a confirmation message to your email address whenever you a Premium payment is made. We'd suggest looking through all your email's folders (inbox/spam/trash/etc.). If it isn't there, try typing "Spotify" or "Premium" in your email's search bar.
If you cannot find anything, try checking the same on any other email addresses you have. When you locate it, you'll be able to find your account's username at the bottom of the confirmation email.