A thermopen is a name given to an instant read thermometer from ThermoWorks.
It's the most accurate thing I have on hand for temperature.
The thermometer I put in my coop has a screw on the back that allows you to adjust the temperature up or down to match up to the known temp.
I adjusted it to match what my thermopen told me, and then have taken the thermopen out with me when I go to the coop to make sure both are giving me the same readings inside the coop, and then take the outdoor temperature with the thermopen.
Different thermometers (sometimes) have different ways to calibrate them. Or not. Surprised how hard it is to find a good thermometer. They're all cheap. Mine that I've got in my coop cost $15. So far seems to be working.