Seasonal eating can be fairly simple. If you use farmers markets as your primary source for fruits and veggies, you'll know what's seasonal by what's being sold. If you use grocery stores, very often price is an indicator where the cheapest produce is what's abundant at any given time of year.
Also listen to your body. I'm not sure which came first - the chicken or the egg, so to speak, but I noticed this winter that I barely thought of fruit except citrus. Then as the weather changed, I started lusting after strawberries, watermelon, and mango. Not sure if that's natural, or just because I started seeing them for sale again.
"Right is right, even if no one is doing it; wrong is wrong, even if everyone is doing it." - St. Augustine