If he had filed a joint return, it would require your signature. He can give you a copy of the return or sign a form allowing you to receive a copy of the return.
His tax information is confidential and the IRS will not release his information to you, without a signed release on an IRS form.
However, you can find some information indirectly. You can obtain information about your own tax records, which would include returns which are jointly filed. If your ex filed a return as "married filling jointly", your signature would have had to have been on the return.
There may be a better way to obtain the information you want. If you have a case before a court, you or your family law attorney may be able issue a subpoena to require your ex to produce his past year tax returns in court. State rules vary, so consult a local attorney or at least inquire at the court clerk's office.