One way to find out if you have been blocked by someone on Facebook is by sending them a message through the website. If the message is returned to you with an error, it means you have been blocked, as Facebook prohibits the exchange of messages with a blocked user.
Another way to find out if you have been blocked is to check if you have any past messages with the user in question. If so, look at the user's name. If it is in bold black lettering, and you are unable to click on it as a link to visit the profile, you have been blocked.