In our studious pursuit of a genetic marker that separates true fighting dogs from wimpy wanna-be fighting dogs, we submit: Exhibit A - the unique behavior of tongue curling, as demonstrated by a bust dog with a 'fighting crop' who lived on a chain with 100+ other dogs. Fighting dog defined!
Clearly the result of a dominant gene, this trait is thought to be passed down from generations of dogs selectively bred for their tongue curling abilities. This suggests that authorities everywhere can begin identifying fighting dogs based on whether they can curl their tongue.
But alas, we let ourselves get too excited too soon. Studies have shown that tongue rolling is not a simple genetic character, and extensive twin studies demonstrate that it is influenced by both genetics and the environment.
Damn. And we thought we were onto something. For more info on the trait that had us hoodwinked: No evidence for a genetic basis of tongue rolling.