The two most common tools are disposable heat press test kits and pyrometer surface probes. The disposable kit is inexpensive and simple. All you have to do is place small test strips down and close your press. After a few moments, compare the colors of the strips to a chart and verify the temperature is accurate.
The pyrometer surface probe is a more accurate and long-term investment. These units work by pressing a metal probe directly onto the platen and reading a digital gauge.
You should not use kitchen temperature reading tools for measuring your heat press, as these have proven to not be accurate and not worth your time.
-Marc Vila, ColDesi