Q: Can I file for defamation against a co-worker who often call me gay, crook and maniac in front of my co-workers?
A: Generally speaking, under California law, slander is a false statement made to a third person that charges a person with crime, imputes him with having an infectious or loathsome disease, tends to injure him with respect to his business or trade (either by imputing to him general disqualification in those respects which the office or other occupation requires or by imputing something with reference to his office, profession, trade, or business that has a natural tendency to lessen its profits), imputes to him impotence or lack of chastity, or by natural consequence of the statement causes actual damage. Civil Code Section 46.
It is important to keep in mind that the First Amendment requires that the statement at issue be one of fact rather than one of opinion. See, e.g., Gregory v. McDonnell Douglas Corp., 17 Cal. 3d 596, 600 (1976).
Whether the statements your co-worker has made would be considered statements of fact and whether they satisfy the requirements of Section 46 with respect to the effect they would have on listeners would likely depend on the context in which they were made. Evaluating the strength of your potential claim is beyond the scope of what we can provide through this service, but you might be able to find an attorney to assist you through the California State Bar: http://www.calbar.ca.gov/state/calbar/calbar_generic.jsp?cid=10182&id=1403.