Why not add a small take-off valve to the main water line? You could open this and have thin, clear tubing attached to give you a level indication. Then you would close the valve so the tubing is not in continuous contact with the water stream.
That's what my relative did. Seems like the most probable option I have.
My handyman worker showed me picture of what looks like a clear gardening hose. I may ask him to buy it for me, and use it the way you described.
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