You can have a dummy box where the probe goes in and set the thermistat to the degree you want it to stay at.
Or if your incubator is good insolated put it in the middle of the incubator and read the ambient of it. If built right you should have a solid temp through out the entire thing.
I have 2 seperate digital gauges that test the INSIDE of the box so I know that the temp there is right. But seems that inside the box and the ambiet are realy close to eachother on my setup. but I only have a small wine cooler incubator.