Orangish brown coolant crud!

About a mo th ago, radiator blew with absolutely no warning. Replaced the radiator,, hoses. Mechanic found an orangish brownish substance, while flushing but doesn't know where it came from. Seem to think substance may have had something to do with the gauge not telling me that the car was hot. One week later orangish is back in New radiator. Is there a way to tell if the engine is OVERHEATING without relying on gauge. Coolant level is good. No leaks and it’s not a head or cylinder issue. How Do I know engine temp, if gauge is suspect???