If you plan to build with bales, you absolutely must have a moisture meter with a probe. It is so valuable that I consider it a “must have,” not a “it would be cool to have” tool.
Use the meter to check the moisture levels of bales before you buy them. I randomly check about 10-15 bales in a stack to see what the moisture levels are. Be sure to check the side of the stack and the top as moisture can get in either direction. Insert the probe all the way into the center of the bale as moisture in the center is almost impossible to drive out whereas some surface water on the sides of the bales can be eliminated quite easily.
You can also use the meter on site. There may be some suspect bales in the delivery you receive and rather than take a chance on installing a wet bale, the meter will allow you to check the moisture levels on the spot and make an informed decision. I keep my meter on site during every job.
Here’s a link to the meter I use. You can buy a meter like this one factory direct, or you can find them in farm supply stores. It doesn’t matter where you go to get it, just be sure you actually buy one!