If the IRS has applied your refund against your spouse's liability, or you're concerned that the IRS may do so, obtain Form 8379, Injured Spouse Claim and Allocation. The form requests identifying information for you and your spouse, and information needed to determine how much of the tax — and refund — is attributable to each spouse. The IRS makes the actual calculation that divides the refund between you and your spouse.
If you're an injured spouse for a return that's already been filed, you should file this form with the IRS Service Center for the place where you lived when you filed that return. If you're trying to prevent the IRS from seizing a refund on a return you haven't yet filed, you should attach Form 8379 when you file the return.