We had a customer recently who was asking how to tell a PST had been configured as a Sharepoint PST, so they could exclude it from some processing they were doing. After some digging, I found the property that Outlook itself uses to determine a PST is a Sharepoint PST, and development gave me permission to document it:
#define PR_ASSOCIATED_SHARING_PROVIDER PROP_TAG(PT_CLSID, 0x0EA0)
To check if a PST is a Sharepoint PST, mount the PST using OpenMsgStore, then call GetProps on the message store object requesting this property. If it exists, you can assume the PST has been configured for Sharepoint. If it doesn’t exist, the PST has not been configured as a Sharepoint PST.