how to tell you have fungus on seedlings?

hi there, i have been growing some cactus seedlings. i read a lot on the web about being careful of fungal attacks due to the humidity in the seed tray environment. I bought some copper fungicide to be prepared but i dont know how i will tell if my seedlings are damping off.
can someone please decribe it to me? also how much copper fungicide would you mix with water for a seed tray 6" square?half a tablespoon?

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