I didn't know either the first time. It was February so we had a heat lamp on em. I would just adjust it in height according to how close they were to it. I think people do tend to overthink things sometimes. I have babies right now; someone on another site was amazed I wasn't keeping a thermometer in the brooder. Ummmmm no. If they are hot they will move back, if they are cold they will get closer. I check on em every so often just to make sure the heat lamp doesn't need to be adjusted. I guess its like feeding them. I know some people say only commercial feeder, don't take any chances. Well commercial feed hasn't exactly been around as long as chickens have. There are people who will vary things more with their own children but they are afraid to do so with their birds. Chickens are pretty hardy animals!
Heat lamps are a pain. We got one of those Brinsea Eco Glows - best thing. The chicks feather out faster and are just all around much better using the radiant heat. Friend of mine uses a Sweeter Heater. Best bit of money I've spent on chicken supplies and greatly decreased risk of fire/lamp dropping into brooder, etc.