How will I know this lead came from the AppExchange?

The lead source codes give you information about how the lead was created and can help you determine how to proceed. The lead source code always takes the form of SFDC-XX|Listing Name or SFDC-dup-XX|Listing Name. If the source code contains -dup-, AppExchange already sent you a lead for this user, listing, or action within the last 180 days. The XX identifies the action the user took to generate the lead.