is_mixed_model(x)

data(mtcars)
model <- lm(mpg ~ wt + cyl + vs, data = mtcars)
is_mixed_model(model)

if (require("lme4")) {
  data(sleepstudy)
  model <- lmer(Reaction ~ Days + (1 | Subject), data = sleepstudy)
  is_mixed_model(model)
}