If they are staying far outside of the lamp range they are too hot (and/or panting and holding wings out). If they are huddled in a pile under the light they are too cold, If they are spread around evenly under the light range and make "forays" outside of the light area for food, water, and play, they are just fine. There is an excellent image for this in one of the threads, I will see if I can find it for you.
The key is this, there is the heated area in the brooder and cooler areas they can escape to if they want to cool off. Keep an eye on them and you will know if they are too hot. Keep in mind that plastic bins or smaller brooders can overheat throughout and suffer swings in temps when the ambient temperature changes (cold or hot room temp).
Use a thermometer if you are unsure of the temp range. The area at chick height under the lamp should be no more than 95 degrees for the first week (lower it 5 degrees each week until the brooder and outside temps are the same or they are fully feathered).