put https://example.com/vms/dry_run
This is the same as the normal Create an VM request however it stops before sending the request to the hypervisor. So no VM is ever created however it does check if the VM could be created.
It needs to be noted that both false positives and false negatives can occur so this is a good approximation but should not be taken as a definite. False positive are more likely then false negatives.