When you send a message to someone, you will see the notification of delivered message or "seen" under your message. In case, if someone blocked you, and you send him a message. Your message will not deliver or seen rather there will remain a single tick.
Although your message will remain on the chat box but it will not be delivered to another person. It is an indication to know that another person has blocked you.
You can also check it by making a call. If you call through Viber and another person seems offline, then it also shows that your number is blocked by another person.