I agree with TwoCrows here. Not having a device to check humidity & temperature is like popping a frozen pizza in the microwave, pressing buttons blindly & hoping for the best. You're either going to come out with a charred disk or frozen pizza. Or in this case, early 'quitters'.
You can find temperature & humidity readers at local garden centers, hardware stores w/ garden sections, pet stores (though generally more pricy), aquarium & terrarium stores online as well. Some gauges are very accurate, but most if not all lose accuracy over time (from months to years, depending on wear & brand). Always good to have back-ups & to make sure they're accurate before starting incubation - that is, make sure your incubator has stable readings 1-2 days before you add the eggs.
Please keep in mind you will likely need the thermometer gauge when the chicks have hatched as well. Too hot & they could pant, fall on their sides, get heat stroke & croak. Too cold & well, they may not be able to huddle close enough.