How do I know if my scissor is really sharp or not?

Frankly, it's not something you can just run your thumb over the edge and pronounce it "Good enough".

Do the scissor's edges cut smoothly from the tip all the way back is a nice objective standard that most people would agree to.  Hopefully anyway, what do I know? :-)

Having the 'correct' test fabric material(s) for the kind of scissor involved is equally important obviously.  Sounds good, yes?

So that's it basically as far as the testing process of GHS scissor sharpening goes.

Smoothly cutting scissor edges slicing through test fabric gives the same hand tension 'feel' as it's butcher paper knife testing cousins.

'Roughness' cutting wise is bad and burrs are always an issue, but 'smoothness' means near factory alignment and all is well.