Cut it into chunks and knock a few of them together. If it gives you a dull thud, they're not seasoned. If it's more like a light click, chances are it's seasoned enough.
I've found that If I cut branches into chunks they season much faster - a few weeks. Perhaps I'm mad, but I've even gotten away with putting green chunks in a warm oven (while it's cooling down for instance). After a few 'bakes', I've used the chunks without incident. Has anyone else tried this? (I've gone through periods where real wood is hard to come by here up North, hence the desparation..)