How do you differentiate fungus infection from any other type on Phal leaves?

also:

many of my Phal leaves have a reather dark color and are kind of flopy.

They get plenty of light and I do not water them unless they are really dry (scratching deep and no feeling any moisture)

Leaves should be light green and have a thick consistency,not flopy,I believe

Otherwise they are doing well. Blooming and growing over 2 or 3 feet!!

any advice? Thanks.

John Salmeron