I realize that this is an ancient thread, but I stumbled across this looking for an answer to this very same question.
The stated "use pvesh" answer isn't entirely helpful; nor is "man pvesh", for that matter. After some sputtering I found that pvesh get cluster/nextid
got me the answer I needed - the next available instance ID (100+n+1).
But I still have no idea how to get a list of all instances that are in my cluster, from ID 100 through 100+n. Maybe this is an odd question to ask...