Install a coolant level sensor. [No I don't know how, but I know one of my prior cars had one.]
Do you have some sort of OBDII scanner which can read real-time the actual engine temperature? I'd think it'd rise abnormally first--gauges are notorious for being slow to respond and/or having deadzones.
2011 Toyota Camry, base, 2.5L/6MT, 187k, hers
2010 Toyota Tundra DC, 4.6L/6AT, 153k, ours
1999 Toyota Camry LE, 2.2L/4AT, 209k, his