We study various tests for the determination of the helicity structure of the P and P′ contributions to two-body scattering amplitudes. Most types of existing data are shown to be incapable of distinguishing between s- and t-channel helicity conservation. Duality tests probably indicate s-channel helicity conservation for the Pomeranchuk contribution, but are inconclusive as far as the P′ is concerned. The only direct evidence for s-channel helicity conservation by the Pomeranchuk contribution is the measurement of the R parameter in the π-p elastic scattering. The conclusion about the P′ depends on what is assumed about the Pomeranchuk contribution.