1. You no longer see their last seen time or "online" status
If you used to see when that person was last online and that information suddenly disappears, it can be a sign. By blocking you, that person stops sharing their last seen time and "online" status with you, even though they keep using WhatsApp normally with the rest of their contacts.
2. Their profile photo never changes or updates
Another clue is that their profile photo stays "frozen." If that person changes their photo from time to time and you stop noticing it, you may no longer have access to their updated image because they've blocked you. Careful: if they never had a photo, or had it hidden from you for privacy, this sign is useless.
3. Your messages always stay on a single gray checkmark (never reach two)
When a message is delivered correctly, two gray checkmarks appear, and if it's read, they turn blue. If your messages to that person permanently stay on a single gray checkmark, never turning into a double checkmark no matter how much time passes, it's one of the most commonly cited signs of a block.
4. You can't make voice or video calls to them on WhatsApp
Try calling them on WhatsApp. If the call doesn't go through, cuts off immediately, or simply doesn't ring at all, it could be due to a block. The same happens with video calls.
5. They don't show up when you try to add them to a new group
Try creating a new group and adding that person by their number. If WhatsApp won't let you include them, or the system doesn't show them as available to add, it could be another sign of a block.
6. Why no single sign is conclusive on its own
Although these signs usually appear together when there's a real block, each one on its own has perfectly normal alternative explanations: the person may have their phone off or out of battery, be several days without data or wifi, have turned off last seen in their privacy settings (which also hides yours), or simply have uninstalled the app. That's why, before assuming you've been blocked, check several signs at once and, if possible, over several days.