Symptoms of digestive issues would at the minor end of the scale not be likely to be detectable by you -- minor belly discomfort, possibly gas or minor bloating, tendency toward constipation as a result of its taking too long to process the food and more moisture being leached from it before it passes, that sort of thing. At the other end of the scale, it should be detectable by you, such as uncomfortable buildup from gasses as food rots rather than being digested properly, painful constipation, possible regurgitation, hesitation to feed.
If your surface temps are almost 100, you might want to try something like using the pad without supplementing with the lamp, increasing the thickness of the substrate, or raising the enclosure a little more above the pad. That's all assuming that the pad is already set on Low, if not that's the first thing to try. The clay hide should definitely be able to do some heat retention, but the type and thickness of clay will determine how long and well it would radiate heat.