There are two methods:

1) With the Gnutella v0.6, most vendors now provide a "User-Agent" field that includes the Gnutella client's name and version, and sometimes a little more information.

2) Monitor any query hits with a HOP count of zero. That's a query hit coming from a client you're connected to. Then you can use the vendor ID, if present, to figure out the client's name. Some clients also provide the version in the extended QHD, but is different for almost all of them.