Nearly all blanks are waxed so it's very hard to tell, even handling them can be misleading unless yup know what it should weigh when dry. often I can tell if something is too wet by holding it as Ive had a bit of practice now, not always though. If I cut up a damp block, I stick it in a thick paper bag with the other damp stuff and leave it to slowly dry out. Sometimes it will still crack but if it's not too bad I'll simply soak it in thin superglue as a stabilizer. It works pretty well and saves a lot of expensive wood from the scrap pile. I also do this to cups that seem like they may crack. I'll cut and sand the cups as normal, then I'll liberally coat them in superglue on the lathe. It soaks in and sets solidly and you can smooth that down and apply a finish to it.
The safest is to get a board which has been well dried, air or kiln. Even waxed stuff that's been around for years can be damp. Boards are also the right thickness to begin with and 4" wide is pretty common.
Talking of damp wood, I got a block of olive last week which still has bark and cobwebs on it which is already cracked quite badly, so i cut it done on the bandsaw and stuffed it into my damp bag in the hope that it will dry without too much more cracking as finding pieces of olive in cup sizes is getting hard and expensive. I'm keeping my fingers crossed on this one, it's a beautiful wood that's well worth the effort and time but boy is it soggy inside.
