Back in the day, if you were setting up a car with non-stock carburetors or a modified fuel injection system and needed to dial it in, you used old-school methods. If it made your eyes water, it was too rich. If it backfired, it was too lean. You could also check the spark plug and tail pipe color for indications of rich and lean running, but these were averages over time, not instant snapshots of a particular throttle setting.
This is a companion discussion topic for the original entry at https://www.hagerty.com/articles-videos/articles/2018/08/06/monitoring-the-air-fuel-mixture-with-uego-sensor