About fire skinks...
Aside from the more vivid colouration, male fire skinks can be distinguished by having a broader, rounder snout and thicker jaw. A fire skink breeder I have communicated with a while back told me that it doesn't hurt as much when her female bites, but when her male bites, he sheds blood. I believe males might also be a little bit larger, though I am not sure about that. And then there is the obvious way to tell, but I don't need to tell you how to figure that out...
Fire skinks need a few inches of moist, loose substrate in order to feel happy and secure. If your fire skink can't completely submerge itself and dig around without being seen, it probably won't be very happy. I would recommend the Eco-Earth brick from Exo-terra, not the Bed-a-beast brick from Zoo-med. Remember that this stuff sucks up water like a sponge so you will need to mist the tank or enclosure thoroughly each time.
It is completely normal to not see your fire skink for several days at a time, but when they do come out, it will probably be in the evening. My skink, Abuto, only likes to come out when it is warm (think upper 80's and lower 90's) and I have just misted her tank. She also likes to wade in her water dish but make sure yours isn't very deep or your skink may not want to actually go in it. Sometimes fire skinks are afraid of pools of water and will only go to their water dishes to drink.
Hope this helps.