Multivariate Statistic: Linear or unimodal relationship?

Hi, if I want to analyze a multivariate dataset, I have to choose between methods assuming a linear relationship between the outcome and underlying gradients (e.g. in PCA or RDA) or an unimodal relationship (e.g. in CA or CCA).

Does somebody know if there are appropriate test statistics which help to decide which kind of relationship could be present? Thanks