I always thought this was kindof a strange question for someone to ask. I guess I could answer it "Her dad is German, her mom is from Belgium and she was bred in America, so whatever that makes her, she is".
It's nice to know some good reasons behind the question, if it is actually someone who knows about the breed. As we all know, there are some people who know absolutely nothing about Rotties who want to act like they know a lot. I actually had someone ask me if our male is a "miniature Rottweiler" (he weighs 120 pounds, and they were serious!), and someone who said my dog wasn't breed standard because she wasn't "brindle".
The list is endless...