One sure sign it is a good light is if the LEDs are CREE XM-L-T6. (some have one, two or three of these LEDS and sometimes more). Three is nice. One is sufficient.
Also, the batteries tend to be a 4 or 6 pack containing lithium 18650 cells.
I have found the headlamp kits to be convenient since they typically have a mount that can be modified to strap to your helmet through the vents.