Can anyone tell me a physical sign that indicates whether a bath is too hot for a pregnant woman?

I know that you are not supposed to take HOT baths during the first trimester (although the birth defect risk is minor), but I do enjoy a nightly bath. I would say my bath starts hot and quickly cools as I add cold water. My bath is never steamy and I don't let myself get to the point where I sweat. However, short of getting a special thermometer, I don't know if it is HOT or WARM. Maybe what I feel is warm is really hot or the other way around. Again - Does anyone know of a physical guideline that I can follow - presence of steam? sweat? something?