did your ex file a joint return or a separate return for you using his address? if he filed a separate return also update your address with the USPS from what he used to your current address. you can also report your ex to the IRS using form 3949-A. in theory, unless there is a joint account he should not be able to cash a check in your name only.
https://www.irs.gov/pub/irs-pdf/f3949a.pdf
