At steady altitude, arm out rested on the coaming of the instrument panel, thumb up aligned with the top of the distant cloud, keep arm rigid and head still for maybe 5 seconds - if the cloud top appears over the thumb you will not clear, if if the tops do not appear you will clear.
Read about it a few years ago, it seemed unlikely - how to keep still etc. but it does work - I use it all the time to fly around the Eastern Caribbean.
The article and AOPA response use very different language. Especially where to put the bottle, the article says "in front of your eyes" but is silent as to distance, while the AOPA response says "at arms length" but no guidance as to where relative to your body. The photo in the article is clearly NOT at arms length. The AOPA response says "water line on the horizon" while the article says "directly in front of your eyes" and makes no mention of the horizon. I don't have an arms length in a C172. Quite confusing instructions in these two sources. At least for me. Could this be clarified, please?
Drink about half, peel off the label, then put the cap back on and turn the bottle on its side. Hold it level directly in front of your eyes, and gaze ahead at the clouds. If the tops of the clouds are visible above the waterline, they’re above you. If the cloud tops are below the waterline, you’ll overfly them. The waterline doesn’t lie.