# How to check if the random effects in a random effect model are uncorrelated with the input variables?

Some asked before "Why do random effect models require the effects to be uncorrelated with the input variables, while fixed effect models allow correlation?"

My question is: how do you check this assumption?

What I did is plot the residuals of the "fixed" effects against the residuals of "random effect"

v <- m.lme\$residuals
attr(v,"std") <- NULL      # get rid of the additional attribute
plot(v[,1],v[,2] )


Then it shows a plot with correlated residuals. Is this the right way to test the assumption stated above? If yes, the assumption does not hold in this case. Is there anything else I can do?