If you used the present perfect it would be quite obvious how they know because you're still doing it :) A) How do you know I'm eating garlic? B) Because I'm watching you do it! The past perfect is used because it something you did in the recent past but the effect of it is still apparent now. A) How did you know I've been eating garlic? B) Because I can smell it on your breath. Sidenote: - It's "how did you know I've been eating garlic", not "how did you know I've been eating A garlic". You don't need the article because garlic is uncountable. When you want garlic to be countable it's either: - A clove of garlic (the individual pieces in the bulb) - A bulb of garlic (the whole root containing the cloves) - Or, I guess, you could say a piece of garlic if you wanted, but it's less correct.