How can I tell if a dog food really uses NZ grass-fed lamb?

The front of a bag is advertising. The ingredient panel is the contract. Start there.

First, look at how the meat is named. "Lamb" or "lamb meal" tells you the species. "Meat meal", "meat and bone meal" or "animal derivatives" tells you a supplier had whatever was cheapest that week. Second, look at position. Ingredients are listed by weight, so lamb should be sitting at or near the top, not fourth behind three plant proteins that quietly add up to more than the meat does.

Then check the country wording carefully, because this is where a lot of bags get slippery. "Made in New Zealand" means the food was manufactured here. "Packed in New Zealand" or "distributed by" can mean the food was made offshore from meat that never saw a Kiwi hillside. Those phrases sit in the fine print near the barcode, so it's worth turning the bag over.

Lastly, ask. A brand that's genuinely using NZ grass-fed lamb will happily tell you the meat inclusion percentage and where the lamb comes from, because it's the good news in their story. Vagueness in an answer to a simple question is information too.