Britney Spears has reportedly hired a hotshot California private investigator to monitor her ex-husband as the couple begins its child custody battle. As Kevin Federline's legal team builds its case by serving Spears' friends and her aides with subpoenas to testify in the custody trial, Spears has commissioned Lisa Javoric to dig up dirt on her ex. Investigators from Javoric's offices recently hit Las Vegas to watch Federline's every move - as the rapper/dancer partied. U.S. magazine In Touch Weekly reports, "The detective watched him intently as Kevin partied in public areas of the hotel. He made a note of everyone who went into his suite." Spears hopes the information her investigator gathers will help her discredit Federline and win her full custody of sons Sean Preston and Jayden James.