How much mana do minions have?

I was considering using Manastorm with Necromantic Aegis, but if my skeleton mages only have like 100 mana then it's pointless.

Is there any way I can check?

The only source of minion/minter stats like that I know of is PoEDB, and I be yet to see them datamine Mana levels. Players test some mods like a Chieftain's charge generation and determined the mana pool/regeneration to cost ration makes it very inefficient without Blood Magic.

In your case I suspect the mages have low mana pools with high regeneration.