If you utilise the Recruitment module within NaturalHR you may want to keep a report on employees that were once candidates in the system, You can do so by creating a report using the Recruitment - Candidates dataset.
Within the Candidates dataset, there is a field labelled 'Employed?' which indicates whether or not that candidate has actually been hired and then made to an employee on NaturalHR:
To take this a step further you could add a filter for 'employed? equals Yes', this way you'll only see candidates that have been hired and made into employees.