I am unfortunate in that our weather is warm and wet with a lot of melt/freeze cycles, and have lots of experience dealing with skiing on sheets of ice. here I am using "Ice" loosely to mean anything from true ice to hard packed snow you need razor sharp edges to stay upright and in control.
Ice and snowboards don't make for a great day out, so avoiding it is important, however any back country boarder or skier needs the skills to handle this kind of unexpected variation. No matter how experienced you are, you can always be caught out and suddenly find yourself on a sheet of ice. Often all you can really do is ride it out till you get back onto snow - depends how skilled you are and how hard the ice is as to what you can do. Don't enter terrain traps such as you described unless you are sure you can safely board/ski them. Presume they are icy till proven otherwise.
Ice, especially if sunny, will have a polished look or a shine to it. Normally there is a transition line where the powder disappears and the ice becomes exposed. The powder can be a very thin layer on top of the ice, in which case you cannot see the difference, and need to read the terrain. If you don't have enough depth of powder to maintain control, but can't see that, life can become very interesting very quickly.
Look for a transition that changes texture and colour, sometime though its very subtle. Often ice tends to have a blueish hue.
Check the depth of the powder at various locations based on the terrain, get a feel for how deep the top powder layer is. If its shallow in one place, it will be shallow (or none) in others. Look for terrain where the wind is funneled - this is where snow will have been blown away. Pick a rounte that puts you on the lee slopes.
The transition from ice to powder can also be challenging, but its usually in an embarrassing face plant into soft snow kind of way rather than a dangerous disappear down the mountain at high speed way.
Only other thing to be very aware of, an ice layer with snow on top is a prime condition for avalanche - if you are dealing with a lot of fresh wind blown powder and coming across ice be very careful of the lee slopes.