We occasionally had lab tests run at golf courses, most notably in my "placement training" (like an apprentice or internship in today's world) year when we were trying to identify and control what was eventually shown to be bacterial wilt called (named then as a result of research at several Chicago area golf courses) C-15 decline of Toronto creeping bentgrass. Dozens of greens died while universities, superintendents, and chemical companies worked very hard to figure it out.
Generally lab tests are not needed and often there is not time. If you have signs or symptoms (and you better know one from the other) of pythium on a golf course you can count on skipping lunch to spray immediately. If you see mycelium you're probably treating for dollar spot (or closely monitoring weather and N). Smoke rings, you're going after brown patch unless it appears temperature and humidity are going to nosedive tonight. Every region has its worst problems. Whatever area you work in you need to know what they are and what to watch for Those plus helminthosporium leaf spot were the big ones here. None of them could be confused with inadequate soil moisture. There is another example: you're not going to spray for helminth in July no matter what signs or symptoms you think you see. Wilt can be spotted from 100-150 yards away and can cause damage quickly. Wind out of the north today? It probably ain't wilt. And yes, we sprayed for wilt: well water (syringe).
Sometimes you do not know 100% for sure. You do the best you can with your eduction, training, experience, and site history intuition. The last step is always "evaluate results."
Lawns and golf courses are different, I realize. Not all golf courses or all lawns are the same or have the same standards, of course. The first item on my checklist when I evaluate a new site or group of athletic fields is which ones are irrigated vs non-irrigated. They are probably going to be treated differently for weeds and fertilizer in my world. Soil type is next or maybe tied for top of the list. Third is intensity of use (game field only or game and practices both held on same field).
I've wandered a little from where I started or meant. My main point is you use all of the information and knowledge at your disposal, call for help if you need it and have time. Lawnsite is a source of help not available when I began. It's great.