Hello, good day and thanks for your question. You may check and try the below and see if you will be able to find or locate the account that you are using.
Go to https://account.xbox.com/settings and login with the account/s that you have to see if you will be able to determine it from there.
Go to https://www.seaofthieves.com/profile and login with the account/s that you have to see if you will be able to determine it from there as well.
In the event that you are still unable to determine it from the above, you can also reach out to the support team at https://support.xbox.com/contact-us as the forum does not have access to user accounts to be able to check this.