There's no easy way to tell other than that since there are numerous variables involved, such as tank size and ambient room temperature. If you want additional heat under the dragon, use an undertank heater or human heating pad set on low. It's not really necessary unless your house gets particularly cold at night. Day time heat should really be provided by overhead heat. Do not use a hot rock.
Hot rocks are dangerous.
Reptiles will continue to sit on rocks that are burning them until they're horribly burned. There are much safer methods of heating your dragon. If you already have a hot rock, the best way to use it is to cut the cord off and use it for a decoration. UVB-Producing Fluorescent Lights Dragons synthesize vitamin D3 when exposed to UVB, and D3 is necessary for calcium metabolization. In the wild, dragons are exposed to the UVB in the sun's rays, but in captivity, especially in colder climates, they just don't get as much sunlight as they need to produce enough D3.