If you have a multimeter then you can compare voltage measurements (carefully, as mains is not to be played with on meter that are not at least CAT-III rated).
If you have some known resistors (e.g. 1R/10R and 1M/10M or more) or something a calcard:
CalCard Ongoing Calibration Verification for Instruments.
calcards.co.uk
Then you can see if the low-ohms reading is sane, and if the high voltage insulation test is sane (obviously if they are 1% or better resistors then you can actually check the calibration, but few folk have them lying around).
Most other things need specialised test equipment, but obviously a simple "does it work at all?" check on a RCD can be done on one of your home sockets (assuming it has an RCD, and you won't upset folk by killing the home WiFi, etc, doing the test). If it trips and give an decent time probably all is well. If not you need a 2nd tester to know if its your RCD or the MFT.
