Ok so, I know that I can call my cousin to Illinois from my Cingular phone, from Maine, on nights and weekends for free, even though he has Sprint. BUT, I just read on their website that I have a limit of 750 off-network minutes. And that if I go over that limit I'll be kicked off. I know for a fact that I'm on their network, but how can I find out if my cousin is? He lives in Waukegan so I dont know. So can you either tell me whether it is or not, if you know, or how I can find out?

You do not have a problem .
That means if you were somewhere during nights and weekends and your phone was roaming and using another network to connect your call As long as your phone is not roaming you are fine.
As far as you cousin his plan takes care of that end of the call