Si(111)-In(4 X 1) is a fascinating quasi-1D system that contains a repeated nanowire motif. The nanowires contain only two rows of In atoms. We demonstrate that it is possible to infer the spacing between the rows from a study of the dispersion of the image state band, measured using inverse photoemission. Our analysis suggest that the rows of In atoms are separated by 4 +/- 0.5 Angstrom. This value is in excellent agreement with estimates of the spacing by both STM and surface X-ray diffraction.