If they are huddling close together, right under the lamp, and peeping, AND if that continues for more than a few minutes, then it needs to be warmer. (Solution: move the lamp closer to the chicks.) Sometimes they will huddle & peep a bit when they are tired and ready to sleep, but they should settle down in just a few minutes if the temperature is right.
If they are moving into the corners away from the lamp, spreading their wings, and panting, then it needs to be colder. (Solution: raise the lamp to move it further away from them.)
And if they move around the box, eating and drinking and sleeping and playing, then the temperature is fine.
It's easiest if you can manage a big enough brooder that part is a little too hot, part is perfect, and part is definitely cool (room temperature is good for the cool part.) Then the chicks can run back and forth--play in the "cold" area, warm up in the "hot" area, relax in the "just right" area.