How can I tell if wood is properly dried?
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.