Residuals from stan_jm

This is probably not the best forum. I, at least, can only offer a general answer, which is to carefully examine the model object. Here's an example from the function's help page; I'd include a reproducible example, called a reprex, but the output exceeds the allowed character count.

# Univariate joint model, with association structure based on the 
# current value of the linear predictor
f1 <- stan_jm(formulaLong = logBili ~ year + (1 | id), 
              dataLong = pbcLong,
              formulaEvent = Surv(futimeYears, death) ~ sex + trt, 
              dataEvent = pbcSurv,
              time_var = "year",
              # this next line is only to keep the example small in size!
              chains = 1, cores = 1, seed = 12345, iter = 1000)