Generally speaking, from having family who were organic market garden farmers who also did a few farmers markets stalls…

Smaller markets tend to be better and more curated/policed by the organizers. This typically means you are driving to a smaller town on the outskirts of a larger urban area. If you are an infrequent buyer this is probably your best bet, plus just the eyeball test.

If you are a weekly shopper, it’s a bit easier to sus out reliably even for the larger markets. Find booths that have the same people staffing them most of the time, who can speak to the product. Look for “ugly” produce - the stuff that looks like it’s made for display at Whole Foods is almost always going to be commercially grown since those varieties are for aesthetics vs. taste. And of course look for booths that generally tend to only have stuff that is in season. Their product mix should shift throughout the growing season in most places.

The latter can by a tiny bit tricky since some local farms are going to have hoop houses and even greenhouses for a few different kinds of veggies. The hoop houses especially are used in part to “time shift” the growing season so as to take advantage of a market where you are a few weeks early or late compared to competitors.

Certainly not perfect but most of the time it’s immediately obvious to me which booths are obviously simply reselling. I’m sure some operations can fake it a bit better, but I would be surprised if I was wrong very often on false positives.