This mod will blast a quite loud audio warning at you when any part of the current vessel is above 70% of its maxTemp. You also get a visual indication with thermometers hovering over the overheating parts. The map view will pulse red.
A critical warning occurs at 90%. You can cancel the audible alarm with an ApplicationLauncher button.
Binary package with assets available from Curseforge here: http://kerbal.curseforge.com/ksp-mods/229928-heatwarning or from http://greenoctagon.co.uk/HeatWarning.zip
To install the binary package, copy the HeatWarning directory in the zip, into your KSP installation's GameData directory.
Thanks to: Stupid_chris, Sarbian, NathanKell, Blizzy, Pizzaoverhead, and all the other #kspmodders who've had to listen to my inane questions and frustrated witterings.