Relevant code:

func (pw *projectWorkspace) settingsPath() string {
	uniqueFileName := string(pw.name) + "-" + sha1HexString(pw.project) + "-" + WorkspaceFile
	path, err := GetPulumiPath(WorkspaceDir, uniqueFileName)
	contract.AssertNoErrorf(err, "could not get workspace path")
	return path
}

// sha1HexString returns a hex string of the sha1 hash of value.
func sha1HexString(value string) string {
	// nolint: gosec
	h := sha1.New()
	_, err := h.Write([]byte(value))
	contract.AssertNoError(err)
	return hex.EncodeToString(h.Sum(nil))
}

So you could get the sha1 hex string of the Pulumi.yaml file then look it up there, but A) That's horrible just use

pulumi ls

B) This is not a supported interface and we'll probably move those files to somewhere more sensible at some point But that's the state of things now if the risk tradeoff is worth it to you.

That should be quite fast

Oh it looks like I linked his fork instead of the upstream