The little wind threads on the sail — the most honest real-time trim feedback on board. When the windward and leeward thread both stream evenly aft, your trim is right.
The fastest route to a real feel for trim costs no equipment at all: sail one entire session watching only the telltales instead of the bow or the shoreline. They show the airflow separating before you feel it in the boat. Reading them is simple. Windward and leeward thread both streaming aft means the sail is pulling. The windward thread fluttering means sheet in a little or bear away slightly. The leeward thread fluttering means ease. Close-hauled this becomes a fine oscillation: head up until the windward threads just start to dance, then bear away a touch. That is genuine pointing — not the stubborn luffing beginners use to force height, which only bleeds pressure out of the sail. Build a fixed scan around it: telltales, water ahead, wind, and back to the start.