General

I would say keep it sort of light and jokey for a while. There's truth in humor, and most people will drop the subject of weight gain or fatness pretty quickly, since it tends to make most people uncomfortable. So the longer you can keep on successfully steering the conversation towards feedism-related topics, the more likely it is that your friend has some interest in it. And if your friend responds with any kind of enthusiasm, that's probably a dead giveaway.

Inadvertently finding someone in real life who's into any of this, even tangentially, would be quite a feat.

You can have some comments prepared to try and use at appropriate moments. Like
-"Yay, food is here! I hate being hungry"
-"This is good. Im not saying it is great food, but food is frikkin great in general, you know?"
-"I hate the Kardashians with their fake asses. You want a big ass? Eat some cheeseburgers."

Just little things to set a tone that you view food positively and aren't anti-fat. Then see if they pick up on that, and echo any of it back to you. If they do, then youcould try being a bit more open.