xenoncrash
Member
There's an icon in the top right corner under the date that will tell you. Also the world just looks different. Lots of lighter colors for spring, tons of green for summer, brown/orange for fall, and snow everywhere for winter.
Spring / Summer
Fall / Winter
I think the calendar will also tell you. Each season lasts 28 days.
EDIT: Also, Pierre will ONLY sell you seeds (he will sell all saplings, though) that can be planted during the current season. So if it's Summer, he won't sell you Parsnips, but he will sell you Melons. That can help tell you which crops to plant.
Thanks very much!