If you can't find out what it is, then enter zero. It won't matter unless you live in a community property state. Income allocation is only really required in community states. Community property states are: Arizona, California, Idaho, Louisiana, Nevada, New Mexico, Texas, Washington, and Wisconsin.
If you do live in a community property state, and there is an issue with your filing, the IRS (and/or state) will notify you of any adjustments (if needed) once your spouse has filed.
Married Filing
Separately in a Community Property State
https://ttlc.intuit.com/replies/3301943
