Ask them about their farm practice. A lot of the growers at our farm market never use chemicals or sprays, but they still have not done all of the "hoop jumping" so they can not SAY organic on their signs. What that means for me is organic quality with poor farmer prices.
If possible ask to sample!! All corn is NOT created equal and a nibble of a raw ear can decide which farmer deserves your money. All you have to do is ask. Most proud producers are glad to have you taste if you are ready to buy. Look for fresh looking husks and yes even worms! If the ear worms are still alive on the cob the corn is fresh and probably not been sprayed (at least recently). Of course too many worms is bad as well. Strip down a couple of random ears to check.
This would get you in trouble in our farm markets where the rule is that "you strip it you bought it" minor pull back at the tassle end to judge the size is usually allowed but it is best to ask first. BTW those ear worms will still be alive and eating a week after the corn is picked so don't use them to judge age. A rule I use is that if it is warm (the corn) search elsewhere. most early morning market produce is picked the night before and stashed in refrig or coolers over night. If the corn is warm it will be starchy not sweet. ~gd
Speaking of corn ... no matter what else you do on market day get that corn shucked and blanched in hot water! As soon as it hits the boiling water the sugar stops turning to starch. Peas are the same way. It will hold in the fridge or freezer after the blanching and can even be easily reheated for supper.
Watch for gluts on the market. If you arrive on market day and simply EVERYONE has a huge pile of green beans for sale you bet green beans will be the bargain of the day.
Shop early AND shop late. If you know you are after fresh tomatoes and it is still early in the season - come early while you can get the pick of the best produce. If you come at the last hour or half hour of market you will often be able to make some good BULK deal purchases. Many times those farmers just don't want to haul their leftovers home and they will give you a great price to just take it all.
Work your work schedule around market day. You are time and money a head if you can get the canner fired up right away and sock away those great summer bargains for a wintery day. Remember some things, like fruit and tomatoes, can just be popped into the freezer straight from market for preserving on a better day. The skins slip right off of a frozen tomato when you run them under some cool water later!
Never be so cheap or drive such a hard bargain that you lose a friend.

Those farm market gardeners can be a wealth of information and a great contact to have.