I came across a downed tree this past fall. Bark was already peeling off it. Wasn’t that large either. So I figured it was as good as seasoned. I cut it up, chopped it into firewood, and burned it this winter. Mistake.
The fire I created with that wood did not produce sufficient heat. Put out a lot of smoke too. It wasn’t as if I hadn't already known that seasoned wood is essential to a worthwhile fire. It was just that my testing of a downed, small, bark-less tree to see if it might be just as good as wood that had been cut, split, and stacked for nine months to a year came back negative.
So unless you have a kiln (I do not) to dry out wood, the only way I have found to get properly seasoned firewood is to do what I just mentioned: chop, split, and stack it this year so it will be ready to burn next year.
And just so you can get a visual comparison of green wood versus seasoned, I included an image with this post.
The larger piece (on the right) with the duller coloration is the seasoned one.
Visual inspection isn’t a 100% certain way to tell green from seasoned, but it can help.