Hello @LastPass User
Welcome to the LastPass Community. I hope I understood your question correctly, if you want to know if you have already used a password on a site, you need to open your vault, click on the pencil icon, and you will see if that password is either weak or has been reused.
Please let us know if there is something else we can do for you.
Greetings!
Heyner | Customer Care
You may also view at-risk passwords in your Vault which LastPass may consider vulnerable by following this procedure.