The various Tuning Grids will help but here are some specific things you can observe that are good signals that will help you know when to change gear (adjust a control line to change sail shape).
Typically the easiest conditions most of us are comfortable in is when the crew is on the trapeze and we can keep the boom in the centre of the boat without crew and helm having to lean too hard.
Boats fully powered, boom should be on centre line, jib barber haulers in board. Crew does the work to keep the boat flat.
Imagine we are sailing in these conditions, the mast is bending perfectly (no Cunningham) and we have just the right amount of leach twist so the sails are setting up perfectly just as the sail maker designed them . The boom is centered and the boat is dead flat. As we watch the Word Champion slip further behind and put the sail-maker back on our Christmas list, we pray conditions will not change. But conditions do change. How do we know when they have changed enough (more or less wind) to make it worthwhile adjusting something?
Increasing Wind .
Your aim is to have the crew low on the trapeze, fully extended and helm hiking hard before you ease the boom, pinch or allow the boat to heel. Any of those things will impact boat speed.