Offset error is the difference between the measured voltage versus the expected voltage for zero input. If you apply zero input and take the fft of that, the level at zero frequency is your offset error. Otherwise, there's no way to extract that information from a non-zero signal. For example, if you have a single-frequency sine wave centered around zero volts, the amplitude in your zero-frequency bin should be zero. But, what if that sine wave is not centered around zero? You'll see a non-zero value in the DC bin; but that's not offset error, that's an actual component of your signal.