This paper proposes an algorithm to determine the top and bottom field order in interlaced video when that information is not provided by the video decoder. Knowing the field order is important for de-interlacing algorithms to reconstruct frames with minimal artifacts. The algorithm works by interpolating lines in assumed top fields and comparing pixel values to bottom lines, calculating confidence values for each field assumption. It then determines if the field order is correct or swapped based on the confidence values. The algorithm needs to run periodically to re-check the field order if the video input is disconnected and reconnected.