You can try the RC nitro trick. If water just sits there, you're closer to 200°, maybe below it, very possible at idle. Water boiling away in about 5 seconds is near 220. 2-5 seconds is something between 250-220. When it boils INSTANTLY, sizzling away like eggs on a frying pan, youre surely over 250. Water boils closer to 230 but if you engine is at 230, the cooling head is a bit lower than that, hence a few seconds for all the heat to be actually absorbed.
You can try the RC nitro trick. If water just sits there, you're closer to 200°, maybe below it, very possible at idle. Water boiling away in about 5 seconds is near 220. 2-5 seconds is something between 250-220. When it boils INSTANTLY, sizzling away like eggs on a frying pan, youre surely over 250. Water boils closer to 230 but if you engine is at 230, the cooling head is a bit lower than that, hence a few seconds for all the heat to be actually absorbed.