I beleive Skillers are lvl 42. These can drop by lvl from a few things in Norm, and almost anywhere in NM as well. NM cows isnt a bad place to hunt these.
Trying to roll specifically for skills IMO isnt a good strategy. I have a friend that wants to do this and i think he is nuts. Sure its possible, but I would rather try and collect those lvl 90+ charms like from Act 5 Super Uniques and Baal and roll for the big stuff and hope the lessor rolls might get me some skills.
There is no way to know "legitly" what lvl a charm is unless a specific monster drops it and you know it and you go look up his lvl and this could be a pain but power to you.
If you specifically want Skills and nothing else then you want the lowest possible charm to get skills as it then cannot spawn the higher lvl mods thus narrowing you possible mods that can spawn. Problem here is that you eliminate the possibilities for the really good ones that might trade for 2-3 skillers