I guess measuring it's weight would be one of the answers, but it doesn't seem practical because wood of a same type can probably fluctuate in it's density enough to make measurement unreliable in 10-20% moisture range. Are there any specific tips for wood like ash, beech or oak?
Your best bet is to get a moisture meter, like this one: Amazon link to one meter. Moisture meters come various ranges: you want one that measures in the 6-14% range or so. Anything above 14% is not useful for woodworking as it will shrink too much.
Meters come in two varieties, pinned and pinless. The pinned meters measure stick two electrodes into the wood about 1/4" and measure the electrical resistance. You tell the meter what species you're measuring, and it uses an internal table to tell you the moisture content.
The pinless variety won't mar your wood, but they're quite a bit more expensive. They use induction or ESP or fairy dust or something to read the moisture level.
If your wallet is glued shut, you can use weight to measure the moisture content. Have a look here.
The meters suggested are very useful, but there are a couple caveats to be mindful of when using them:
- They can only measure the moisture level to a certain depth. If you have thick lumber the center may have higher moisture content that what you read near the surface. Different meters read to different depths.
- They can't indicate if the wood was dried "properly" as you ask in the title. For example defects such as case-hardening, checking, or other internal stresses can be caused by improper drying techniques but would not become apparent until you cut into the wood.
You might want to check out the excellent book Understanding Wood by Bruce Hoadley from your library.
If you really want to be sure there are moisture meters you can purchase. Just do a google search for "Wood moisture meter" and that'll tell you what the moisture content is.