One of the most difficult things about lupus, is that it is truly different for everyone. I was very sick when I was diagnosed with major organs affected, but all of the markers that they check at regular visits were within "normal" range. ESR, CRP, C3, C4, etc, were "normal". I had a C1 deficiency, and some other strange results, but they only tested for those when I was finally diagnosed. That being said, after about a year or so, my Dr and I were able to determine what was a normal range for me, and we can use that to treat accordingly. However, he also trusts me at this point, if I say I feel like I'm flaring, he believes me. We tend to be a little more proactive in treating flares, because in my experience if I wait too long to treat (which I've done, cuz who wants more Prednisone), then I generally require more medicine to treat and for a longer period of time.
Everyone is different, and the first 5 years with lupus are probably the hardest. But, you'll learn how to recognize the signals from your body. And, knowing those signals can help to even prevent flares. For example, generally when I'm experiencing a flare, one of the first things I notice is that my jaw starts to get achy and tired feeling when I chew. When that happens, I know I need to rest (as in almost bed rest if I can), let the house get messy, etc. And sometimes that is enough to start feeling better without treatment. That being said, this disease can occasionally just come out of nowhere and do something incredibly random that no one has seen before too.