You can identify if someone has blocked you on Facebook Messenger through several telltale signs. First, try searching for their profile by name—if you can't find them and previously could, they may have blocked you. In Messenger itself, if you attempt to message someone who has blocked you, your message will fail to send, and you'll typically see an error message or the message will remain in a "pending" state indefinitely.
Another indicator is that you won't be able to see their profile picture, last seen status, or online status in the chat. If you open an existing conversation with that person, their profile may appear blank or limited. Additionally, you cannot call them through Messenger, and any attempt to do so will fail.
However, it's important to note that some of these signs can also indicate other scenarios: the person may have deactivated their account, deleted Messenger, or changed their privacy settings. A definitive way to confirm a block is to ask a mutual friend if they can still see and message that person. If your mutual friend can interact with them normally while you cannot, this strongly suggests you've been blocked.
Facebook doesn't notify you directly when someone blocks you, so you have to piece together the evidence yourself. The most reliable indicator remains the combination of being unable to find their profile, failed message delivery, and lack of profile information visibility.