
You'll want to see them exploring the entire brooder freely. If they're all hanging out directly under the lamp, trying to snuggle together for warmth, they're too cold and you'll have to lower the lamp and raise the temperature for them. If they're hanging out as far from the lamp as they can get, maybe even panting with their beaks open, they're too hot, so you will need to raise the lamp to lower the brooder temperature. If they're avoiding one area of the brooder like the plague, you may want to check to see if there is a breeze in that area of your brooder, and move your brooder or set up another brooder guard to eliminate the breeze.
Make sure it is no hotter than 95° in the hottest part of your brooder. Not only do the chicks need nothing warmer, but heat lamps can be a danger not only in sparking chicken coop fires, but are also a fire hazard in brooders because they're dusty and bedding is flammable. Make sure to double and triple check your safety, or consider using a safer heater, like one of these. A broody hen is even better!