Is there a way for Pulumi to determine which resources it doesn't manage? We're migrating some existing infrastructure over to being managed by Pulumi (it was originally setup manually, without any IaC), and want to make sure we haven't missed anything (for reference, we use GCP, but I assumed that if Pulumi has this functionality then it'd be generic)

Maybe consider applying tags to the stuff you've imported, e.g.

managed-by = pulumi

, then query for anything that doesn't have the tag.