If you try to add the businesspartner and it already exists there'll be an indication/error in your soap response:
<?xml version="1.0"?><env:Envelope xmlns:env="http://www.w3.org/2003/05/soap-envelope"><env:Body><env:Fault><env:Code><env:Value>env:Receiver</env:Value><env:Subcode><env:Value>-10</env:Value></env:Subcode></env:Code><env:Reason><env:Text xml:lang="en">1320000140 - Business partner code 'KXXXX' already assigned to a business partner; enter a unique business partner code</env:Text></env:Reason><env:Detail><Object>2</Object><ObjectIndex>1</ObjectIndex><Command>AddObject</Command><SessionID>97091400-833E-4518-B0DA-2404C740F281</SessionID></env:Detail></env:Fault></env:Body></env:Envelope>
If you want to do a check using DI server before sending your add message there is a data retrieval message callled "GetByKey" as well.
Check the SDK.chm documentation in your SDK folder.