There are a couple of indicators that you may be blocked:
You can no longer see a contact's last seen or online in the chat window. Learn more here.
You do not see updates to a contact's profile photo.
Any messages sent to a contact who has blocked you will always show one check mark (message sent), and never show a second check mark (message delivered).
Any calls you attempt to place will not go through.
If you see all of the indicators above for a contact, this could mean that the user is blocking you. However, there are other possibilities. We have made this intentionally ambiguous in order to protect your privacy when you block someone. Thus, we cannot tell you if you are being blocked by someone else.