Its pretty hard to do without some equipment. The prefered way is to look at the adaptation values and test to be sure they are not something else.
If an AMM is the cause of your problem it is pretty bad. The adaptive values should be near the max. Lower partial around 1.25 or above.
The actual output voltage can be read but a 10% skewed AMM would be about impossibly to condemn from the results and it could cause such problems.
Bosch Master, ASE Master, L1
33 years MB technician