Two ways.
1. Change the configuration in Google Voice to "Show my Google Voice
number as caller ID when forwarding calls," add your Google Voice number
to your iPhone contacts, then set a different ring tone for the Google
Voice number. But you won't know who is calling you since the CallerID
will always show as your Google Voice number instead of passing through
the CallerID of the number that is calling Google Voice.
2. Don't change the configuration in Google Voice and instead go through
your iPhone contacts list and set a ringtone different from the default
ring tone) for your contacts. Spam calls, which will be from numbers not
in your contact list, will use the default ring tone. I think that this
is a better option, but more work initially.
You can also turn on spam blocking in Google Voice by requiring calls
coming from numbers that are not in your Google contacts to have the
caller announce themselves. This stops many spam calls.