I live in Greece and only buy seasonal produce because everything out of its season is choke full of pesticides, plus they don't taste the same. For example tomatoes in September are sour, while if you buy them during February they're sweeter. You can also tell by their shape, produce grown in a greenhouse looks in a far better shape than the one grown in the field, because the latter is exposed to weather condition and it takes a beat on its surface/appearance. As a general rule of thumb, the more uniform a vegetable/fruit appears the less tasty it is.
Other than that, there are plenty of guides on the web regarding what to buy when. Just make a search for "seasonal vegetables and fruits" and you'll find a lot of infographics.