Eric Legault [Outlook MVP] answered my question on another site/forum:

Use the PropertyAccessor to check for FInvited (http://schemas.microsoft.com/mapi/proptag/0x00620003); will equal true. Does not exist otherwise.

So, my Function to tell if a meeting has been sent is simply:

Function MeetingSent(oAppt As Variant) As Boolean
Dim PropName As String
Dim oPA As Outlook.PropertyAccessor

PropName = "http://schemas.microsoft.com/mapi/proptag/0x00620003"

'Obtain an instance of PropertyAccessor class
Set oPA = oAppt.PropertyAccessor

'Call GetProperty
MeetingSent = oPA.GetProperty(PropName)

End Function

Thanks Eric!