How to diagnose a fungus problem

Can someone give us noobs a little help, maybe a link? I'm trying to diagnose an issue with my plants and I have been told I might have a fungus or mold issue how can I tell if that is my problem? I've been looking for an online guide but all I can find is info on nute defs? Symptoms are yellowing leaves with brown spots and slowed growth. some of the leaves have some faint white dots... is it mildew and how can the issue be resolved before the plants die off?

pics can be found in my post " I need some advice"...

How often do you water? Do you see white fuzz? Are you growing hydroponically? read this https://www.rollitup.org/marijuana-plant-problems/488004-guide-nutrient-deficiency-toxicity.html. Fungus and mold is a killer, look up how to get rid of mold on cannabis plant or something hope that helps