Block checkers do not work anymore. You can find more information about that at
Block checkers DO NOT work.
The 'block checkers' that you find on the Internet are very simple, really. They just add the contact you wish to check. If he appears online to the block checker and offline to you, he has blocked you. However, it's a very insecure way of checking: he might have blocked all others to see his status (block checker will always show offline then), or he might seriously be offline, ...
/ 