Most employees hired in a retirement covered position before January 1984 are covered by the Civil Service Retirement System (CSRS). Most employees hired in a retirement covered position after January 1984 are covered by the Federal Employees Retirement System (FERS).
To determine your retirement plan look at your most recent SF 50, Notification of Personnel Action in Block 30. The 1-character code represents your current retirement plan and a definition of the code.