How do you determine if cut firewood is dry enough to burn?

I find the easiest way is by weight, or by how easily it cuts when you chop it into smaller pieces. Dry wood is significantly lighter than green wood and chops much more easily. If you are still not sure, try throwing one log on the fire. If it causes a lot of smoke and you see sap fizzing and bubbling out of the ends, it is still green. :)