Auxin can be bio assayed by:

Plant growth regulators are also called phytohormones. There are two major classes of phytohormones, growth promoters and growth inhibitors. For every hormone, there is a typical bioassay, which depicts its typical activity. Avena coleoptile curvature test is the bioassay for auxins. This was shown by Went in 1928 on a decapitated coleoptile which showed the auxin-like activity.