How do you tell if your cat is acting lethargic?

I think my cat B.A. is being lethargic, but how do you tell?

When the vet asks me if he is being lethargic, antisocial, and shy, I don't know what to say. B.A. is ALWAYS lethargic, antisocial, and shy.

I don't want to have anymore fifty dollar vet visits (like a recent visit with George) where the vet tells me the cat is tired and allergic to pollen.

I think he's mostly referring to a change in behavior. If your cat has always been a lump, that doesn't necessarily indicate ill health. But if he used to be a tornado and now he's a lump, that's cause for concern.

Thanks, Maybe he's depressed.