Add a Script (Python) to the Zope root folder.

I called mine isProdInstalled.

It has a parameter of productname.

# check which plone sites have a product installed. 
# Product is is hand entered when running the Test tab

from Products.CMFCore.utils import getToolByName

objects = context.objectValues()                        

for object in objects:
    if object.meta_type == 'Plone Site':
        portal = getToolByName(object,'portal_url').getPortalObject()
        qi = getToolByName(portal, 'portal_quickinstaller')
        if qi.isProductInstalled(productname):
            print productname, "is installed in:", object.id

return printed

From the ZMI, when you select the Test tab, you will see a box asking for the productname, enter something like PloneFormGen and you should see the Plone site IDs that have this product installed.

If you want to run from a URL, you must pass the parameter: something like this:

    http://127.0.0.1:8080/isProdInstalled/?productname=PloneFormGen