Most of today's established names in the business did exactly that. A private detective can be self-employed and work alone, or as part of an agency. Today, with the proliferation of various professional security service agencies, there are choices of entry at various levels. Specialist skills can also play a role in securing a suitable position. Some of the important agencies in the business have also been set up by retired defence or police officers.
Typical training is provided by the employers of security personnel at various levels.
Eligibility requirements vary but physical and psychological requirements, prior related work experience are deciding factors for acceptance. In-house training is expected for new recruits at all levels. Although meant to provide security to clients in many ways, being a private eye is not a secure profession. There is always an element of danger and uncertainty, the unknown quantity of what an investigation may unearth. Advancement opportunities and growth are not really defined, but those who have proved themselves remain in demand and can quote their price.