You can send a GET request to the /account-details endpoint to receive general details for a linked account. To find out if an account successfully linked (opposed to knowing if it successfully synced), you can reference the "status" field in this request. There are three possible statuses you can get back in this field:
COMPLETE is returned when the account is successfully linked!
INCOMPLETE is returned when the account cannot be successfully linked, which usually happens when permissions are invalid or some aspect of the authentication details were entered incorrectly.
RELINK NEEDED is returned when the authentication expires, which means you need to re-enter your account credentials in the linking flow. This status will not resolve on its own unless you re-enter the credentials.
You can view a sample response for the full endpoint in our Developer Docs.