OK I will answer this one knowing that someone will jump in and correct me..
put the thermometer in water that has lots and lots of ice.. it should read about 31F 32F or close..
the hygrometer can be tested:
get yourself a gallon sized sealable plastic bag. take a cup and fill it almost full of salt. add water to make a thick paste.put your hygrometer into the bag .
put the cup with the salt in it into the bag and seal it.. wait at least 8 hours and check the reading.. it should read 75%..
if for instance, it reads 80% .. then that actually is 75%.. there is a difference of 5%. so for every reading you would have to subtract 5% with that hygrometer.
and if the reading shows less than 75%.. then you would have to add the difference..