How can I tell if someone has blocked my number?

You don’t. There is no notification that your number has been blocked by the other person. There are, however, signs that that it has. When you call you are instantly sent to voicemail... every time you call. You might also get a recording that "this number is not taking calls at this time” but that can be caused by other things as well..

There is no way you can know if you been blocked. It's a person's right to block anyone they don't want to communicate with. If they don't ever respond to you, it might be a good guess they've blocked you, but you'll never see a notification saying you've been blocked.

You have likely been blocked if you always get a recording when you call and leave a message, but you never get a return call, or you send a message and never receive a reply.

On the other hand, it is always possible that you have not been blocked.....and you are simply being ignored.