I am not really sure if it’s profitable to produce and use 1000 LL counterfeit bills but there are plenty in circulation apparently. I don’t think I’ll ever bother to check if a 1000 LL is counterfeit or not and I doubt that merchants/shops will do.
Nevertheless, here’s how you can verify that your 1000 bills are not counterfeit:
via MTV
Try to get your hand on one and review it