Well, for diagnosing it in veg you need to inspect the root tips CLOSELY. Sharp and well defined equals ok, mushy equals systemic rot in most cases. Tips only in veg, the rest looks just fine...
And there are a number of ways to keep it partially under control to save crops, depending on your system and available remedies, and your personal preferences. Search, here and in Google.
Remember that different strains has different tolerances to it, and it even differs between the individual plants in the same strain due to genetic diversity. There are also a multitude of pythium strains and sub-strains, with different capabilities for persistence and devastation.
And also, some claims that it is curable, and some claims that it is not...
I argue, based on mine and others experience and scientific litterature, that in the majority of cases it is NOT. But confusion in this matter easily arises from the fact that it has a veg-dormant/bloom-active life cycle, and some plants are more resistant and some pythium strains milder. And finally, the individual growers varying success in suppressing it with the applicated treatments or "cures" or implemented "preventative measures", may in some cases lead him to believe that it is cured, when infact the outbreaks are only effectively prevented .
Just make sure to keep it the fuck out of your plants in the first place, which is relatively easy with some basic knowledge, experiencence, and sound gardening practices.
And yes, pythium is everpresent in soil and water as dormant spores, and WILL activate and attack plants that are susceptible to it due to moisture stress, low O2 and/or other factors. Then, with ONE sick plant in your room, it will most likely spread to the others as aggressive, live, active pathogens with a very high disease-pressure. As opposed to the dormant, everpresent low-disease-pressure spores before given a chance to take hold...
Good luck.