What about ncvTest() ?
https://stats.stackexchange.com/questions/58941/ncvtest-from-r-and-interpretation
This is test for homoscedasticity/heteroscedasticity. If this assumption is not met (homoscedasticity) it will indicate non-linear relationship ?
This is helpful as well:
https://forum.posit.co/t/how-to-check-the-linearity-assumption-in-survey-package/155990/8