Thanks for your response but trust to 3rd party in not an issue if it's the fastest way I'd use it. I want to track the appearance of the 494784 block as soon as possible in order to sell my BTCs at the next moment.
I assume you hold them on an exchange, which means you don't need to track the appearance of the new block, but you need to track when the exchange does that.
Blocks can always orhpan, so it's never instantly certain the block is final.
An easy way to check it, is watching
http://bitbonkers.com/ on your screen. Turn on the volume and wait for the block to fall.
I see no reason to quickly sell right after the fork BTW.