Employees are covered by CSRS and have social security coverage because of a break in service greater than 1 year. Indicated on your SF 50 (Notification of Personnel Action) by a C in box 30 and the notation FICA and CSRS (Partial).
This page can be found on the web at the following url: http://www.opm.gov/retire/tools/calculators/ballpark/CSRSOffsetpop.asp