... To know if it has happened? *he thinks for a moment*

It is difficult to really say, as it is still a very rare occurrence. There is no absolute when it comes to it. Sometimes, anything connected to it will vanish as well. Sometimes, they might still linger, regardless of it.

Unless if one has the ability to freely move through time at such precision, it is hard to say if a future has vanished or not. ... Especially when you consider your previous statement of being in an alternate past.