In some cases, you won't find what you need in public records. Physical surveillance of the subject will be needed.

For example:

• Divorce investigations often require physical surveillance as a means of documenting infidelity.
• Insurance investigations may need surveillance to prove fraud.
• Whenever a subject's lifestyle is under investigation -- e.g., if questions have arisen regarding drug or alcohol abuse -- surveillance may be called for.

Surveillance is the bailiwick of the private investigator. It's not something that should be attempted by amateurs. For one thing, it can be dangerous. Beyond that, surveillance must adhere to legal guidelines and must be performed by a professional who's experienced at it in order for it to be successful.