You can check whether a virtual machine has any baselines with unknown compliance status attached to it and start a scan.

Prerequisites

Verify that you are connected to a vCenter Server system.

Procedure
1

Retrieve the compliance statuses with the value Unknown for the baselines attached to the VM virtual machine and store them in the $statuses variable. 

$statuses = Get-Compliance -Entity VM -ComplianceStatus Unknown
2

Check whether the virtual machine has any baselines with unknown compliance status attached to it and start a scan. 

if ($statuses.Count -gt 0) {
Test-Compliance -Entity VM -RunAsync"
}