Concerns in testing a potentially water damaged Macbook battery
If you have the Apple Service Toolkit, that is the easiest way, if you don't... there aren't may programs out there really suited for this type of work. 

If you are showing that the battery may be holding or attempting to obtain a charge, the battery probably will be OK to be used. I am sure that enough time has passed in where the battery has been completely dry.  In many cases you could submerge the battery, dry it off and it will work. The batteries are pretty much air tight to begin with. 

If a battery had a crack in it, or something of the sort you'd know.  

The best thing to do is to attempt to get a charge out of it, if the battery holds one you are fine, if not then you may be able to go down to an Apple Store and ask if there is a way to view how much life is left in the battery.

Of course, this is assuming the battery hasn't been saturated in salt water either, if it has, you may just want to throw it away.  Otherwise I think you will be safe to use it as long as it holds a charge that is.