Baal charms are the common name for chars dropped by Hell Baal. This means, your nightmare Baal charm isnt "it"
Yes, they have a special ilvl, those from Baal and Nihlatak only.
There is no way to see the ilvl of items.
So, how can you tell? You cant. Trust is the way to go.
Reason to why people want Baal charms is that with their ilvl, they can get better mods than other charms, for example, you can get a skillcharm with up to +45life, instead of +40life for most other hell charms.
Also baal SmallCharms are wanted, since also they can get highly-wanted mods not availible elsewhere.
If you play single player and drop a Baal charm (or any other charm) into your ATMA stash, you'll see the item lvl when you view it. Baal charms are lvl 99. This is the only way I know to determine a charm's level.
Seffe, you don't really believe that asking others to use a hack isn't any better than using it yourself, right ? I'm sure the mods don't care about the difference.
Baal charms in their broader sense are charms with an item level being high enough to get any affix, so they are particularly useful for the rerolling recipe. With respect to GCs, only Baal (L99), Nilathak (L92) and Diablo (L94) can drop them.