With a little practice the transitions from ripple to glassy and back again can be managed by feel, and with eyes outside the cockpit there are often subtle height clues to be seen on a glassy surface:
Do you train this for water flying? (I sure hope not). Trying to low fly glassy water by "feel" is going to result in the feel of hitting the water - which may end badly.
The subtle height clues are as often
misleading as clues, and often are not at all there. The nature of glassy water is that it can go from "clues" to "no clue" in an instant, and at that instant you suddenly can't visually detect if you're going up or down. The same will occur over unbroken snow, which I was teaching during dual ski flying training last week.
When I am training water flying, or ski flying, upon arrival over a glassy or unbroken surface with no "shoreline" cues, the pilot will either continue in a "glassy water" landing technique (which cannot be power off), or they will begin a climb away, or, they will not be signed off. It's worth the over dramatization, and is not negotiable. I see it as being every bit as critical a piloting discipline as stall/spin recognition, as the stall/spin
might happen with altitude and time to recover, failed glassy water/unbroken snow awareness very certainly will have
zero opportunity to recover.