When a person is blocked on Twitter, the blocking account becomes visually similar to an account that has set its tweets to protected. Only when a blocked user attempts to re-follow the blocking account does the Twitter blocked account message appear.
More subtle signals that an account has been blocked is that the blocking account's tweets or mentions no longer appear in the interactions or timeline sections of Twitter. Twitter blocks on a per-account basis, so users who are looking to subvert active blocks can make separate accounts to re-follow the users that have blocked their previous accounts.
A second account can also be used to verify if a user has blocked an account. If the second account is able to view the user's tweets and activities rather than seeing a protected tweets page, then the first account is confirmed to be blocked.
Users who are looking to avoid blocking other Twitter accounts can choose to set their own account to private. This allows the account owner to choose which followers to approve or reject, removing the need for blocking in the case of spam or harassing accounts. Private users are also protected from non-followers, as none of their Tweets and timeline activities are publicly viewable.