How does one know??

[CrimsonReaper]

How does one know that he/she can perform the reaper death seal when u're technically dead once using it??? XD

 

[Alastor]

The same way they know they can use Rinne tensei.

 

[Tarinth]

That's what I also wondered about the 8 Gates lol. And another handful of techniques.
But the simple answer is... Kishi logic.