"The best way is to install a proper temp gauge and use a thermometer to check its accuracy. "
Yep- buy a 110deg thermometer for $10. Then idle the motor from dead cold to the maximum it gets to with the rad cap off and the thermometer in the top of the radiator. You can slip a sheet of newspaper down in front of the rad to help it as the motor isn't under load.
Note down the temp and time it takes, and you will get a curve. If the temp gets to 100deg you have a problem, the cooling system just isn't up to it anymore. It should show cold, then quite quickly go to 75deg or so, and slowly climb to maybe 80 odd. Then you will know what temperature the marks on the gauge really mean.
In pouring rain it shouldn't get hot at all.