The importance of testing the sub-floor for moisture before your hardwood flooring is delivered to the job site cannot be overstated.

I've seen it time and again where perfectly good flooring is ruined because the homeowner or flooring contractor deliver the wood flooring before ensuring that the installation area is fully dry.

Small amounts of moisture, not detectable by the eye, are enough to cause significant expansion in the new flooring.   Later, when the flooring dries out because of normal house heating, it will shrink leaving unsightly gaps between the boards.
 
Use a moisture meter.  Rent one if necessary.
  • Plywood should read no more than 12%.
  • New hardwood flooring should read 7 to 8%.
The difference in moisture content between hardwood and plywood should be no more than 4%.

FYI - if you actually test your hand with the same moisture meter, you will get a reading in the high 20's. This gives you an idea of just how little moisture is needed to affect the wood flooring.