Unfortunately, there's no way to be certain if another user blocked you. If you don't see a read receipt but the other user responds to your message then you can be certain that you have not been blocked, the other user has the feature turned off. However, if the other person doesn't respond there's really no way to tell one way or the other. That's because when a user blocks you, you can still see their messages in groups. Some users have stated that when they attempt to message a contact who has blocked them, the message disappears after a few minutes. But, we weren't able to replicate this in May of 2021.