The first thing to do is to print the annual general summary (see here).
What needs to be checked are the totals of the gross salary, the social insurance bases and the withholding taxes.
You have to be able to justify the differences between the figures (normal if it is correct).
Typically, the difference between the gross and the AVS base (in the above example, 2'400.-- corresponding to an dii and accident) and the other insurances.
For the record, the difference between the base xxx and the salary xxx :
- base = original amount
- salary = what the contributions will really be calculated on (taking into account the ceilings, age limits, exemptions, etc...)