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 paypall or 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.