To answer both questions.

On the no UV lights, These frogs spend 90% of their lives burrowed up to there eyes in the substrate waiting for the next meal to come to them.
So no sunlight gets to their skin.

On where to place the sensor, the best place is on the opposite side of the enclosure of your heat source and about in the middle from the top.

If it is a humidity / temperature sensor , avoid misting the sensor or it will not function properly.