in VMAN I suppose the best way is to use "vm.internal.dateCreated" attribute. Query may look like "vm.internal.dateCreated:[now-1days TO *]" to get list of new VMs in past hour.
There's also a trend "VMs new to SolarWinds in the last 7 days" which shows you count of newly added VMs in time.
Let me know if it's helpful or if you need anything else.
Thanks, this is exactly what I was looking for!