Due to privacy and security reasons,
Facebook Messenger does not have a specific tool
or feature to let you know if someone has blocked you.
You can infer that you have been blocked
by checking on a few things,
but these do not guarantee
that you have indeed been blocked.
Looking at how the Messenger app behaves
when you send a message may give you clues
that you have been blocked.
When you use the Messenger app on your phone
or in a browser,
you may be familiar with the status icons
next to the messages you send.
There are four such icons.
An unfilled empty circle
means the message has not been sent.
This is typically because you don't have
any internet access when you try to send a message.
An unfilled icon with a check mark
means the message has been sent,
but not delivered to the recipient.
A check-mark icon that's filled in
means the message has been delivered.
An icon that's filled in
with the recipient's profile picture
means it has been read.
If the recipient is logged on to Facebook,
your message should be delivered within a few moments,
and you will see the check-mark icon.
Once the recipient views your message,
the icon will change to the message-read status
with the profile image.
If you send a message to someone
and the message is not delivered,
meaning that only an unfilled check-mark icon appears,
it generally means one of two things:
The user has not logged in to Facebook,
or the user has blocked you on Messenger.
It's not always possible to tell the difference,
but if you see that the recipient has posted
to their Facebook page
or comments on other people's posts,
that implies that they're logged in to Facebook.
If your message remains undelivered,
you're probably blocked.