Not quite sure, but the editors just released a list of their favorite apps on Sunday (they do so every Sunday, they call them Apps of the Week). One of the apps from this weeks' favorites is an app called Ampere. I'm not sure if it'll tell you if your phone is charging wirelessly, but you might give that a try. It should tell you at least that it's somewhat charging. Come to think of it, the battery bar in your top right corner should say it's charging, or so would the lock screen at the bottom (it says "Charging" and usually gives an ETA on full battery, but since wireless charging is so slow it may just stay at "charging")