THURMAN'S FAMILY LAW TIPS:
OBTAINING THE OTHER PARTY'S INCOME TAX RETURN WITHOUT A BATTLE
Well, maybe - one can hope that the battle can be avoided. Family Code section 3664 specifically authorizes you to obtain the other party's financial information without having to file a court proceeding, except for noncompliance, once each year. This may be strategically quite wise, not to mention possibly far less expensive, because your assumptions about changes in the other party's income may not be correct - and filing a blind motion to modify child support or terminate spousal support can blow up in your face in the sense that the amounts you receive under the present orders may wind up being reduced instead.
Unfortunately, if the other party is like so many family law litigants - completely obstinate - you will want to read my comments to FC sections 3660 to 3668.
CALIFORNIA FAMILY CODE
MODIFICATION, TERMINATION, OR SET ASIDE OF SUPPORT ORDERS
Discovery Before Commencing Modification or Termination Proceeding
Family Code Section 3665
(a) A copy of the prior year's federal and state personal income tax returns shall be attached to the income and expense declaration of each party.
(b) A party shall not disclose the contents or provide copies of the other party's tax returns to anyone except the court, the party's attorney, the party's accountant, or other financial consultant assisting with matters relating to the proceeding, or any other person permitted by the court.
(c) The tax returns shall be controlled by the court as provided in Section 3552.