how to tell you have fungus on seedlings?

[Sam]

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?

thanks

Sam