Shoutbox

There is no sure way of checking if someone has blocked you by putting your name in a box and clicking check, but there are many ways that you can check if you're blocked...
1.    If you have Stuff Plug-NG, and someone opens your window when they are offline, a good chance is you're blocked. Messenger Discovery has a feature designed to detect and tell you when an "offline" contact opens your window

2.    If you haven't blocked "All Others" in your account settings they can make an alternate account to check if you are really online.

3.    The most simple one, when you suspect blockage, ask someone if they are online on their contact list when they aren't on yours

There's loads more of little tricks and stuff that could indicate if you are blocked than just that the contact could be using...