How to tell if someone has blocked your number?

Person A is through AT&T carrier, and Person B is through Verizon. It is suspected that Person A has blocked Person B's number. Person B sent an iMessage to Person A, but it does not say "Delivered" under the message, and the message did not go through as SMS and the message remained blue as an iMessage. Additionally, when Person B calls Person A, it goes straight to voicemail with an automated answering service prompting you to leave a message. Has Person A blocked Person B (via the blocking option on the iPhone or through the carrier service)?

Added (1). Person B's iMessage on the Messages app on Apple laptop show that the message has indeed been delivered, but still does not say "Delivered" under the message on the iPhone. What's going on?

If they blocked you then all your messages will say delivered to them, but they never actually receive it.