You would need to compare you last year return to this year return.
Did you make more this year than you did last year? If so, you may not qualify at all for the premium so you have to pay it back in its entirety. If your household income is less than 400% of the Federal poverty level, then you may have to pay back some but not necessarily all of the credit you were not eligible for that you received, but if it is more than 400% you would have to pay back all of it.
Are you married filing separately this year? If so, you would generally not be eligible for the credit.
If your income changes at all during the year, or if your living situation changes, you should report that to Healthcare.gov as soon as possible to avoid having to pay too much back.