> Well I'd like to keep fighting but Sol would say something if he realized Mithrail was a celestial and possibly change his battle strategy so i need to know why Joseph knows that she is a celestial

Joseph is actually something of a genius because he's spent his whole life reading. He only cares about gaining knowledge, and nothing else. He's read plenty of descriptions of what Celestials are like, so he can pretty much tell based on the information he knows.