Mostly I didn't know. I know when the problems started and looking back I did see a sign that didn't click at the time--my DI canisters started to deplete much faster in late August.
In general it was just doing the normal things, water changes, testing and reduced feeding, to correct the visible problem--hair algae, cyano and sometimes diatoms. When doing what should have fixed those problems didn't work and my testing results were wacky--zero phosphates, medium to high nitrates I kept searching. Eventually I read something about what happens when the activated carbon phase expires on an RO unit and tested my RODI reservoir for ammonia. Bingo! Mind you it was 3 months of diligence on the other measures to little effect. Experience will make you more observant to the right signs. If my DI resin ever starts to kick off again I will know something is up.
Learning from Erin (Kensington Reefer) I have learned to be observant which of my corals suffer first when phosphates start to rise, which ones respond poorly to nitrates, which ones can't tolerate a calcium drop, which ones freak out when alkalinity is too high, etc. I don't test every parameter every day and I don't think it would help if I did. I do spend at least 5 or ten minutes looking for signs of change though and try to be observant about unexpected behaviour.
When you can correlate changes in your tank to what you can quantify then you are one step ahead of the game--because once you can do that you can react to changes immediately on observation. Testing or other quantification will confirm it but noticing early and reacting is key. You'll just get more experienced as you go.
I happen to be cautious and cheap (ahem, frugal). This is may be a good mix for staying in the hobby for a long time. I want to advance my tank by building on success rather than just trying things randomly. I don't buy $100 or $500 fish or corals without confidence that I won't just immediately kill them. In fact I don't buy those animals at all because I don't have that confidence. I am getting more confident with LPS corals even though recently killed several chalices.
It seems like a good choice to start with cheap (often free) and easy to care for corals and fish even though it's nobody's intention to buy things just to kill them. Then when those survive and flourish reward yourself for success with more interesting and expensive critters.
I realized I had reached a new plateau when I simply discarded unwanted discosoma mushrooms and kenya tree props. It probably wont ever happen with too many other other corals but it was recognition that my abilities had advanced to a definite point. You'll get there too with patience and careful observation.