I don't believe autotuning exists in 1.3.5, it was only added to 1.3.6 - sorry :-/

Also, is there a way to check that auto-tuning is working normally? Is dbmon.sh the right way?

The error log at startup will tell you what the server sets the caches to.  ldapsearch on nsslapd-dbcachesize will also return the adjusted size if I am correct, but I haven't tried it.  But like I said before... this feature is not present in 389-ds-base-1.3.5