Where can you find the information that is needed to measure an indicator? Sometimes, the information is available from external sources, such as government statistics or studies made by universities, consultants and so on. Often, you’ll have to find the information yourself, through questionnaires, through observations by project staff, by means of your monitoring and reporting system.
Verification sources have to be able to provide information that is readily accessible, reliable and up-to-date. This last element is often a problem with official statistics.
As for reliability: don’t believe any number you come across in a report or in (official) statistics. It’s wise to combine and compare information from different information sources. Sometimes the available information may not be specific enough. For instance, a national average may hide important differences between different parts of the country.
The means of verification determine to a large part the cost of your monitoring system. Information may be difficult to find or to get access to. It may require a big effort in terms of human resources and/or time to get the information that is needed. Again, if it becomes too costly you should probably reconsider your indicator(s) or your monitoring system as a whole.