Hi everyone,

How do you moisture test subfloor if there is already hardwood below? Do I rip up a board or two? Thanks!

There is a special attachment that plugs into a moisture meter that has long prongs. The attachment has a driving weight that slides along a rod so it can force the inch and a half prongs through the finished floor and reads at the tip of the prongs using the right meter.
Lignomat has a tool that plugs into a miniLigno C that will tell the moisture content up to 20%. Above 20% is not necessary to have an accurate measurement since wood can start to rot above 19% moisture content, unless you are using it for a kiln drying valuation.
The sub-floor measuring device does leave a couple of small holes, but saves taking up the finished floor. The holes will need to have putty to cover.
I think other companies also have a device that can be driven into the sub-floor.
You may be able to measure the sub-floor moisture if you can get at it from below, maybe crawl space or through the finished ceiling below.