I've figured out from fishing around on here that the fruit fly looking things that are flying around my plants are called fungus gnats. The problem is that I have about 25 different plants under a plastic greenhouse, and I have no idea which has the gnat problem. Of course, an issue there is also making sure the gnats don't start nesting in another plant as well ...
I have heard that soaking the plants in soapy water should kill the gnats, but I'm concerned about how I'm going to pull that off. Most of these are tropical indoor plants which can't handle the very hard water we have up here, and I'm not sure how to soak them in soapy water without damaging them in some way.
So my question is: is there a good was of identifying which plant(s) actually have the gnat problem, and also: what other methods of getting rid of these pests do I have?