Deadheading crew member reported ice shedding on the wing upper surface of B737NG during climb out.

Narrative

We landed in LAS after a 5 hour flight. Airport conditions were overcast with some light rain; above freezing (about 3-4 degrees). I realized that we had the potential for some ice formation; so I asked the First Officer to look for ice 10-15 minutes after our arrival time. I left to buy us both some lunch. After my return; we were still experiencing rain; and the First Officer reported that she had recently performed a preflight and that she had noticed only the acceptable amount of fuel frost on the underside of the wings and observed no ice on the top. I took her for her word and we departed on schedule uneventfully. We flew to our destination and eventually diverted due to a low ceiling at our original city. During deplaning; a deadheading crew member informed us that he had observed clear ice on our wings after departure; and most especially on the left wing. He told us that this ice eventually broke off during the climb. I am concerned that our procedures to detect ice by looking through the passenger windows and from the ground might not be enough in all lighting conditions. After dark; being extra cautious with wet wings; I asked a mechanic to use a stand and feel atop the leading edge on the left wing. He obliged; but I sensed that this was not a routine request. To be sure; my mag light was helpful in the dark environment. However; in the overcast light in LAS; this would not have helped. Frankly; I'm not sure what can be routinely done that would make our procedure 100% reliable. Ask Boeing. Is there some paint or some other method that would make clear ice easier to observe/detect? Perhaps we should deplane and open the emergency exit hatch(es) when maintenance isn't available and under certain conditions. Where maintenance is available; should using the 'stand' method be a normal function? The Operations agents reporting to us that deicing is ongoing for wing ice have been helpful to me in the past; but not absolute.