How can I make sure that the PRONOTE base is empty before launching an initialization?

From the Client, go to the menu Configuration > About NameOfTheBase.not then click on the tab Main data: if the data counters are not all at zero, make a copy of the base before launching the initialization: you can then re-import this data using the command Imports/Exports > PRONOTE > Recover the data from a PRONOTE base.