There are ways of telling how dry the wood is such as knocking 2 logs together and getting nice hollow sound, rather than a dull thud, or observing how many cracks are appearing in the log end. Well cracked will normally suggest, well dried.
However, the really best way to check the moisture content is by using a moisture meter. This will give you a pretty good guide as the actual reading of the firewood, both inside and out. Check inside by splitting a log open. Just be aware however that moisture meters are not always 100% accurate, particularly the very cheap ones.