A: You cannot know for sure. Signs include your messages showing "Sent" but never "Delivered" (RCS) or calls going straight to voicemail.
The easiest way to check is to open the conversation thread with that person. If the "Block" option in the menu is unchecked, or if the "Unblock" button is no longer visible at the bottom of the chat, the number is no longer blocked. how to unblock a text message on android