There are a couple of indicators that you may be blocked by one or more of your WhatsApp contacts:
You can no longer see this contact's "last seen at.." or "online" status in the chat window.
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).
BUT there is a better way, just download Whatsapp Block Guesser and you will be able to know if you're blocked.
