Nope....at least I never do. Well, I take that back....very early in the Mama Heating Pad thread @azygous asked me what the temperature was under there. I didn't know - all I knew was that the chicks that had been using it for several days seemed happy. But out of curiosity I put a wireless thermometer in the cave. The room temp was 69 degrees. The temperature under the heating pad was 82.9. I know, GASP, SHUDDER!! Why weren't all my little chicks dead, or huddled up in a corner shivering? Well, it's because they were being warmed by their backs coming into close contact with the underside of the heating pad. I wasn't warming the entire space they occupied. The heating pad was the warm spot, not the air in the cave. Others who have used MHP and have taken temperature measurements have reported about the same temps I came up with.
A much more accurate measure is how the chicks behave. Are they running all over the place exploring and eating? When they go under do they make little churring sounds rather than distressed peeping? Do they climb on top to survey their domain and take little cat-naps? If so, they are doing just exactly what they'd do under a broody hen and you've got it made! If they act like they are cold, huddling and cheeping loudly, they are cold. Either pop the temperature of the pad up a notch or smoosh (official MHP technical term) the frame down a bit to bring it closer to their backs. If they won't go in at all, chances are it's too warm. Turn it down or pull the center of the frame higher. Easy as that!