which instrument-panel warning light would show that headlights are on main beam?

The instrument‑panel warning light that shows your headlights are on main beam is the small blue headlamp symbol, usually shaped like a headlight with several horizontal lines pointing straight ahead.

What the main beam warning light looks like

On almost all modern cars:

• It is blue.
• It looks like a headlamp icon with straight, horizontal “light beams” pointing to the left (in UK‑style graphics).
• It only illuminates when your high / full / main beam is switched on, not just side lights or dipped beam.

A simple way to picture it: if the dipped‑beam symbol is green with lines dropping downwards, the main‑beam one is blue with lines pointing straight out.