To tell if it is PM or not, the smell is distinct at least to me. You can give em a good sniff, instead of awesome fresh marijuana smell, think musty odor, or clothes you forgot to put in the dryer after 3 days.
For treatment it really depends on the number of plants and size. For larger plants I go straight to Myclobutinil spray if I'm at least 4 weeks from harvest. It is a system and you can get away with out a total drench. If you are lucky and have small enough plants, I would dip in the same stuff mixed in like a 5 gallon bucket. This method knocks out PM, and treats systemically for 15-20 some days. Clean the area around your grow as well as you can and keep humidity down.
There are many contact treatments you can use, however In my experience they only worked as a band-aid buying me a few days at a time. The Myclobutinil when I used it knocked out the PM and 3 months later, not a hint of it.
Cheapest source of Myclobutinil is from Home Depot/Lowes for about $15 in a concentrate. Branded Spectracide Immunox . Immunox Plus is another product that has pesticides in it. The pesticide is actually a synthetic Pyrithin(sp) I beleive, and I avoid the pesticide product. However it is the same pesticide some people have used for Spider Mites and other bug problems.
This is the same active ingredient as Eagle 20, which comes in a huge concentrate and is about $50 online or at some hydro stores.