How to check the linearity assumption in survey package?

regTermTest does this.

Provides Wald test and working likelihood ratio (Rao-Scott) test of the hypothesis that all coefficients associated with a particular regression term are zero (or have some other specified values).

“Linearity” in this context means that coefficients of the regression are not of quadratic or higher order. β0+β1x rather than β0+β1x2 .