Interesting question, so let's spend a minute thinking about it.
I think we can agree that all animals must eat to survive, and that hunger is a driving force in making that happen.
But when we talk about food drive, are we really talking about hunger? If yes, then why use yummy treats? When I'm hungry I don't need a bowl of ice cream; I need the most filling food possible. If a dog works for food out of hunger, then any food will do. And since I'm not a deprivation trainer, I have no intention of allowing a dog to go hungry in order to get behaviors. I happen to find hunger more uncomfortable than physical pain - if I have a choice between a few shots or going without food all day, I'd take the shots. With that in mind, I will not withhold food to get my dogs to work out of "hunger". I will, however, ask a dog to work for a meal that I plan to give to them anyway, if they are willing to work for it. If not, they'll still get it.
So what is usable food drive? We're talking about love of food - ice cream and snickers, not bread. Most of my dogs enjoy delicious food. They will eat pieces of cooked chicken even if they've just finished a huge meal. That is because they are eating because they like food, not because they are hungry.
A dog that will eat tasty treats even though it's not hungry - that dog has usable food drive for training. Most dogs fall into this category.
A dog that will eat any food even though it is stuffed - that dogs has a very high food drive.
And a dog that only eats when it's very hungry or in it's cozy house with no distractions? Does that dog have usable food drive? I'd argue "no".
Some dogs eat regardless of what is happening and others lose all appetite when they are excited.
That's Lyra. She likes food well enough but any competing alternatives will negate her food interest. If I starved her I'm sure there would come a point when she'd work for food even with competing interests, but why would I do that? Is that humane? I don't think so, especially if I can find alternatives that keep her in the game. Honestly, if you're training for sport, it makes no sense at all to train a dog that has to be subjected to chronic deprivation in order to work. That dog isn't cut out for competition, and that's perfectly fine.
Some trainers have never seen a dog who lacked food drive. I'm surprised by that because I see it rather often - maybe 10% of the dogs I train do not have usable food drive.
If you've really never seen it, take a look at this video. For background, this dog is young; about a year of age. We have tried just about every food known to man; from kibble to tripe to cheese, chicken, beef, etc. both cooked and raw. In this video, we show her favorites sitting on the floor - there for the taking. This includes dehydrated tripe strips, freeze dried lamb lung, and a popular cookie. As you can see, she is not overweight. At seven pounds, there would be serious ethical/medical concerns about not offering food for an extended period of time. On the morning of this session she had not been offered breakfast, so one would expect that she would be hungry. She has been training for several months, and is comfortable in the room. She does know a few behaviors - touch, up, sit, down, a bit of heeling, etc. Fortunately she has some interest in work for the sake of work, so the work causes the eating rather than the other way around. Offering food "marks" approval rather than rewarding her.
She eats enough to keep her alive. She eats to live; she does not live to eat. It's important for dog trainers to recognize that this is a real phenomenon rather than suggesting that the owner just hasn't found the right food or withheld enough meals. Finding alternatives to food is a necessary skill for a trainer, and probably the hardest thing I need to do.
In this video, you'll see that this dog could care less about the food laying around the room (which happens to include the food that is in the box). What eventually captivates her and gets some work is the idea of "food as contest"; chasing the container, and curiosity about what might be in it.