How I checked mine with a oral thermometer, I covered a cup with saran wrap (press n seal actualy) poked the thermometer into it and let it sit awhile. Not effected as quickly as the air in the bator rom the heat element cycling. Gave me a consistent 99.7 temp (forced air).I use an oral thermometer that everyone has in the cabinet to set the temp of incubator. They are accurate. If a digital then check the high and low to average. About 10-15 seconds after heater turns on is the lowest temp on mine. If not many eggs in incubator the range can be 96.5 to 102.5 but that averages 99.5 which is spot on.
For non fan incubators the temp should be 101-102F measured at top of eggs.