Business Partner to Service Agent?

Hello,

there is the possibility to check if a Party (business object Party) is related to a Service Agent?

I tried using the cross BO association "BusinessPartnerRelationship" in the "BusinessPartner" association:

Party.BusinessPartner.BusinessPartnerRelationship

but this association always returns an empty collection!

Also I tried querying directly the BusinessPartnerRelationship business Object but unsuccessfully.

There is another way?