To know if someone has blocked you on WhatsApp, look for these signs together as none alone confirms a block:

• You can no longer see the person's profile picture or it has turned to a default avatar.
• You can't see their "last seen" or "online" status in the chat.
• Messages you send show only one gray checkmark (sent) but never a second checkmark (delivered).
• You cannot see updates to their WhatsApp status.
• WhatsApp calls to them do not go through.
• You cannot add them to a new WhatsApp group.
• Tapping to enlarge their profile picture may make it disappear if you are blocked, unlike just seeing it in chat where it might be cached locally.

These indicators together usually mean you've been blocked, but some could be due to privacy settings or connectivity issues. WhatsApp does not send notifications when someone blocks you, so you need to check these signs manually.