There’s no surefire way to know if someone has blocked you, but there are some clues you can put together that will give you a pretty good idea.
https://www.techradar.com/phones/ios/how-to-know-if-someone-blocked-you-on-imessage
https://www.techradar.com/phones/ios/how-to-know-if-someone-blocked-you-on-imessage