Thus far they've not found any DEM ballots pitched into rivers/lakes or mail employees/gov't poll employees or politicians (like TX, PA, just to name a couple incidents and there are many) nixing out ballots of Republicans. That's not to say it hasn't happened but mail carriers mostly democrats -- I've heard this from 3 postal workers in my state - 2 DEMS, 1 NON-DEM.
There is no organized check-system in place, of course. No ballot identification in NV and I'm sure same in many other states. Everything else in life demands verification but not the most important -- our votes. There's a huge reason for this.