Sometimes you can go by smell! Produce that smells good also often tastes good.

I also have some luck going by looks, but by specifically looking for exemplars that have flawed looks (in ways that I know don't negatively affect flavour). For types of produce where you can sort of tell which ones are good, other people tend to go for the ones that look good but also don't have blemishes. That selection process results in the good ones being statistically overrepresented among the ones with flawed looks.