You don't need to know if it's a notification. Non MMS enabled clients wont react to the message, only the MMS enabled phones will understand the SMS. The MMS client in your terminal will fetch the MMS message accrding to the instructions in the notification WAP push message.
Teemu / Forum Nokia
RE: when i get a sms ,how can i know it is a notify of a MMS
Generally speaking, the notification SMS contains information in some special format. This includes a "port" number indicating that it will be forwarded to WAP protocol, and an embeded URL indicating the actual position of the waiting MMS.
It's the MMS Client's job to explain these informations, and go retrieving the MMS according to the URL.