How do I trust that I am typing my password for Google when I'm using a Safari web view in an any iOS app?
I agree with Greendrake.
With my experience with iOS you cannot verify if the source is from google or not, unless (like Greendrake said) it is in a browser window/interface.
However, if you have decent knowledge in reading packets then there is another solution. There are third party applications that will allow you to view the packets of your iPhone when it is tethered with your computer. From there you would be able to view if the authentication interface is from google or a third party.