With the right hint of Anne Walther (a.k.a. 'awello'), I could find a solution:
from Products.CMFCore.utils import getToolByName
def myMethod(context):
qi = getToolByName(context, 'portal_quickinstaller')
prods = qi.listInstallableProducts(skipInstalled=False)
for prod in prods:
if (prod['id'] == 'your.productname') and (prod['status'] == 'new'):
# further code...
For whatever reason and fortunately, the status of a product during a re-install will return 'uninstalled', not yet installed products come back with status 'new' and finally already installed prods of a site shout out loud and proud: 'installed'.
This way it is possible to distinguish a reinstall from an initial install.