If you have access to a good machine shop, they should have a set of Jo blocks or other standards that they use to check their measuring tools. Another possibility is an instrument calibration or metrology service in your area who should have the capability to check measuring equipment. Even very expensive equipment should and for some, must be periodically checked for accuracy. If you have standards that you can use to know the error, if any, of the Harbor Freight tools, you should be ok with them for light use.
A machine shop with a surface grinder could make 1", 2" and 3" standards for you. I have not seen the Harbor Freight micrometers but in all likelyhood they, like any, are adjustable to get an accurate zero reading.