Sodium is, I think, hard because I don't think it color-changes with any safe chemistry, if any chemistry at all.
Most sodium test kits are based upon conductivity. But, one needs a baseline for that, e.g. water, 50-50 ethylene glycol mix, etc. One would also need to test at about the same temperature, as there might be a big difference between a cold day, a hot day, and after the engine warms up. And, one would also likely need to be able to rule out accumulated contaminants, e.g. metal particles, etc.
Using a conductivity meter seems tough as a practical solution. Although, I guess if one tested frequently at a somewhat stable room temperature -- any fast change could be cause to take notice.
There are some meters that test for sodium ions, often used for food or medical applications. But, they aren't cheap -- I think they are all $300+. Things like this might work:
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https://www.amazon.com/HORIBA-LAQUAt.../dp/B00BSXGYVC
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https://www.amazon.com/HORIBA-320068.../dp/B076F94K33
Cheers!
-Greg