Diesel ,compare your temp gauge to a known thermometer. Starting cold with the radiator cap off, try running your engine up to temp in the shop with a digital thermometer in the radiator neck. You don't need to make it boil just bring it up and compare your gauge , the thermometer and at what point the thermostat opens (you'll see circulation in the top of the radiator). You should see circulation of coolant before the time your gauge redlines. If your guage redlines before you see circulation and/or you hear popping/gurgling sounds in the cooling system , shut it down ,let it cool, pull the thermostat and put it in a pot of water and bring to boil to see how and at what temp it opens if at all.

Be careful