If you suspect that someone has removed you from their iMessage contacts, there are a few tell-tale signs to look for. First, open up the Messages app and try to send a message to the person in question. If the message sends as green instead of blue, this is a sign that you've been blocked. Additionally, you won't be able to see when the person was last active or whether they're typing a response - another dead giveaway that you've been cut off.
Of course, it's possible that the person simply hasn't enabled iMessage on their device, in which case your messages will always appear as green. In this case, the best way to confirm whether you've been blocked is to ask the person directly.