how do i know if i have been blocked

You actually can't know if you have been blocked or not unless someone who does know (likely the other party member) tells you. The reason for why the message isn't delivered yet could pertain to the other device not being connected to be able to receive the message at the moment.

Blocking is set up so that the person being blocked can never determine they are blocked.

The only way you would know if you are blocked is if the person blocking you decides to tell you.