I check the soil temperature with the type of cooking thermometer that has a thin pointed probe. What felt barely warm to me turned out to be almost too warm for the seeds.
The description for one seed-starting mat said it didn't have a thermostat but was designed to keep the soil temperature 10 degrees above the surrounding room air.
The ordinary heating pad I have gets too hot to set the seed tray directly on the pad. I have to elevate the seed tray an inch or two above it as Granny said. I finally got tired of messing around with that setup and invested in a real seed-starting heat mat.