The K-1 should track your capital account in Section K (bottom left), depending on the method used there, it may be equal to your tax basis. You can also track through a basis schedule which some preparers will attach to the K-1. If you don't have tracking through your K-1, you need to go back to your records and record what you contributed, income, losses, and distributions to determine your basis (you can probably google a spreadsheet that will help with this). As noted above, other more unusual items may affect your basis as well, and don't forget about nondeductible expenses.
In the end, if your distributions are less than your basis then you can report a capital loss. If it is the opposite then you'd have a capital gain. You'd report it on Schedule D.