How do I get income verified so I can renegotiate child support? My x is newly receiving a SS check in addition to other income.

Per a final divorce decree, if my x-husband's gross income increased above a certain level, he was supposed to let me know so we could renegotiate his child support. I am aware of his filing for Social Security benefits and starting to receive them since late 2008, plus with several job changes, it is likely that his gross income is above the specified level. With college expenses looming for our teen child, I would like to pursue getting him to resume the child support that he stopped paying two years ago, at a minimum. Is there a way that the court or I can find out what my x's income is currently? I would appreciate your assistance.