The only way to know is to measure the temperature on the floor of the tank above the mat with an accurate thermometer (the plastic dial and cardboard strip ones are notoriously inaccurate). Most mats will get well over a Corn's safe maximum (which is around 90 degrees).
A human's core body temp is above 90, so if you put your hand on the mat and it feels warm or hot, that's a rough guide that it's too hot. I've run a test setup without a thermostat and the mat hit 120 degrees before I unplugged it.
It's a difficult thing to consider, especially when you've had no problems so far. It's good that you're thinking about the issue. However, that doesn't mean there won't be problems in the future. Personally I wouldn't run a heat mat without a stat to regulate it. Corns need surprisingly low temps - I always thought of them coming from hot steamy Everglades. But of course, Florida has winter, hurricanes and torrential rain as well.
If you can afford it, it's nice to provide the best possible conditions rather than just a basic setup that seems to work OK without really considering the comfort of the snakes.