What things can a passenger look out for, to indicate an emergency?
Honestly, as a passenger, you're not really qualified to look for problems.  If you're a pilot qualified and with experience in that type then you might see something.

I've had passengers tell my flight attendants that they saw flames coming out of a seam in the engine cowling.  It was actually a section of orange rubbery material that was sticking out and flapping in the breeze.

Ice on the wings depends on the airplane too, you're often going to see some ice on the leading edges in icing conditions, and it's normal.  To know what's not normal, you have to have experience with that specific type.

There is actually one thing you could probably look for.  There are no airplanes that I know of that can safely take off with ice on the top of the wings (behind the leading edges).  That would be a legitimate thing to scream about; however, a pilot that would take off in such a case probably shouldn't be a pilot.

You can certainly ask questions about things you see, but don't phrase it such that you think you found a problem.

Window shades are requested to be open for take off and landing by some operators to allow the flight attendants to quickly assess the situation outside when directing an evacuation.  It has nothing to do with allowing passengers to look for problems.  Not all airlines have this policy either, it's somewhat uncommon, the first thing I do when I sit down at a window in the back is close the window, and it's rare that I'm asked to open it.