Log In Page email / password

I have a Log In page in my appgyver app, My app has the firebase authentication.
I need to add a condition " If the email is not the same as the registered one in firebase" show a toast message “email incorrect” and same for the password but I don’t know how to do the logic to make it work, comparing the email that the user put with the one that is registered in firebase.
Need some help pls

Check out this video around 18:05. This is a more generic message but seems to accomplish the same general purpose.