how to make sure website does not have copyrighted images?

one of my former clients wants me to go through his entire website to make sure none of the images have been stolen (i.e. none of the images are copyrighted) because he doesn't want to get sued.

I have been thinking about how to do this, but can't figure it out. what's the best way of doing this? The website is not that large, about 60 pages, and I estimate around 500 images.

would I just go through each image and plug it into google reverse image search? then what? once I plug it in, how…

how to make sure website does not have copyrighted images?