Layer mask just shows solid when channel is selected. Cannot see red overlay just solid, set to 50% but shows 50% color not screen. NOTHING WORKS. Spent 2 hours with adobe and they "don't understand the question" even with demos and screen shots.
There's a couple different things you could be talking about...
1. An alpha channel in the Channels panel: In the Channels panel, toggle on the visibility for the Alpha Channel and you should see a red overlay. If not, something is broken.
2. A Layer Mask attached to a layer in the Layers panel:
To view the B&W alpha mask attached to the layer alt-click (option click on mac?) the thumbnail of the layer mask.
To view a red-overlay version of the layer mask, alt-shift-click instead.
3. Quick-mask mode:
Press Q to enter quick-mask mode. It will show your current selection as a temporary, editable red-overlay mask. Press Q again to go back to normal editing with the new selection based on your quick-mask.