Google Images gives you the ability to search images across the web. If you click on Advanced Search, you will see content rights and use permissions.
Google, from a legal perspective, does not determine what is a licensed image and what isn't. That decision is left up to the owner of the image. They do, however, do their best to help you determine what the owner of the image intended. They do this in a number of ways. Chief among them is the fact that they encourage webmasters to indicate in the code of image what it's license is. Here is their guide on that issue:https://support.google.com/webmasters/answer/114016?hl=en
That said, just because Google says an image isn't licensed does not necessarily mean that it isn't. It should give you some confidence that it's licensed or not, but ultimately that decision is left up to the owner of the image. If you have any question about a particular image, you can contact the webmaster directly. One good way to help you determine that (outside of Google's characterization) is to go an image lookup on Google and see if that image is used lots of other places. If it is, then there is a good chance it's unlicensed.
We recommend licensing through a stock service to ensure your images are properly licensed. If someone contacts us to remove copyrighted content, we normally take the content down and ask questions later.