B737 Captain is informed after landing that ice was not completely removed by the deice crew prior to departure.

Narrative

It had snowed three inches the night before and all the aircraft were completely covered. The Deice Crews worked for about a half hour on each aircraft to clear the snow and ice. All of this was done in the dark and there was no falling snow for the entire deice and taxi out. The Deice Supervisor completed the operation and confirmed that we were clean. We taxied out and departed. Upon arrival; a pilot in the back from another carrier informed me that there was still ice on the right wing; even after an hour flight. As it was now daylight and I was able to get a good look at both wings. In fact; there were small sheets of ice on both wings. They were still almost invisible; even under daylight conditions. They were composed of clear; smooth ice that blended in with the wing surface. I could easily see how they would have been missed when covered with a layer of wet glycol. I called for a deice crew to complete the removal before departing. This is a case of everybody doing everything right and still not succeeding. The only way to see this ice was after it had dried - a condition that was neither possible nor desirable with glycol deicing. The best response is to educate our ground deice crews that this could happen under rain-turning-to-snow overnight conditions.