You don't.

Sadly, this is one of the most insane quality of life improvements I've yet to see SE touch base on. the only way to PERSONALLY learn the caps is to level every single craft to 50 so you can attempt to meld every item you want to test on, then add materia till you hit cap. It's an incredibly poorly designed upgrade system and I'm shocked SE thought it was ready to be released.

That being said, the only way people figure stuff out is by trial and error. Also, some people have taken the liberty to post on the comments section of items at xivdb.com indicating the stat caps. Other than that, there is no way to tell.