By default, the IRS will assume you are 50/50 owners, and you each should report half the capital gains. If audited, you can rebut this presumption, but you need to have records to show them. For example, if you paid 50% of the down payment and 50% of the mortgage payments until 2017, but did not make mortgage payments after, your ownership interest has been diluted over time. Or, if the other owner took a cash out refinance and spent the money on themselves, that would also dilute your equity.
Also, remember the proceeds are not your gain. If you determine that you are 30% owner, your gain is 30% of the selling price minus 30% of the adjusted cost basis. That might be more than the proceeds, especially if there is an outstanding mortgage.
The IRS does not have to award any adjustments that you can't prove with adequate records. So if you are audited, you need to show them records and a reasonable argument/calculation about why you own less than 50% of the property. Keep your calculations and records with your other important tax papers for at least 3 years (6 years is better).
