Genius, say hello to Motion to Compel!
Going on the presumption that your divorce was not a straight forward Summary Dissolution, you can file in the family court a Request for Production of Income and Expense Declaration After Judgment in lieu or with an eye to filing of a Motion to Modify Support.
In this situation, either party has the right to serve on the other a demand for a completed Income and Expense Declaration together with copies of the last federal income tax return. If he should drag his feet and ignore your request, you can then file a Motion to Compel Compliance with the request.
So you do not need to hire a private investigator as the income and expense declarations are made through and supported by sworn affidavits, so anything less than the truth as best known to the party will be perjury, and then that party's problems shall deepen considerably to the point that the paying of mere alimony will be considered a cake walk in the good ol' days.
I do not know what state you are in, but you most probably be required to do this using standard judicial council forms which your local family court should have for sale for nominal fees.
fredrikklaw