Question: How would one say "How good are you...?"

Question: How would one say "How good are you...?"
I know it's Gaano ka..., but I don't know if it's "kagaling" or "magaling". For example, if I were to ask someone in Tagalog about how good their English is, how would I ask that? Is it "Gaano ka kagaling mag-english?" Would "Magaling ka ba mag-english?" also work? This is one of those situations where I get the mechanics, but can't seem to get it exactly right.