The ALMA band 9 (600–720 GHz) receiver is a dual channel heterodyne system which is capable of detecting orthogonally polarised signals utilising a wire grid beam splitter. Two Superconductor–Insulator–Superconductor (SIS) mixers mounted behind hybrid mode corrugated horns are coupled to the 12 m Cassegrain antenna via a wavelength independent configuration of two off-axis elliptical mirrors. We outline an approach involving accurate physical optics simulations in conjunction with precise experimental measurements of the complete optical front-end which guarantees the highest performances. This practical verification approach can be generalised to all quasi-optical receivers to validate system performance. In this paper, we verify the optical design and estimate antenna system efficiency. Comparison between measurement and simulation indicates precise information is achievable in estimating system performance allowing potential improvements in ALMA instrument calibration accuracy.