You can try to take the best out of Google to find out if your organization is having a phishing sibling, but it won't probably show you all the phishing websites.
If a take the
allinurl example, and you're looking for similar website as paypal, if the attacker choose a name like
paypol, this won't be seen in the Google result page. Now, this can pop up some good one, as you can see on this interesting link (that answer a bit of your question too).
You will have more success searching with the
intitle parameter, like :
https://www.google.fr/#q=intitle%3A%22PayPal+Merchant+Services+-+Payment+Solutions+for+Your+Business+-+PayPal%22 because they will make the phishing website look identical to yours, so you will have more chance to find them using the title tag.
Now, this won't never be 100% efficient.
A good alternative is to use this service, PhishTank, which list all the phishing websites reported by users. But again, if no one reported a malicious website, you won't find it.