This is the answer you're looking for. You can crawl your site with the Screaming Frog SEO Spider and take a look at all the outbound links: https://www.screamingfrog.co.uk/seo-spider/
You could also check all outgoing links with something like Integrity (Mac): http://peacockmedia.software/mac/integrity/free.html
I use Integrity link checker on my sites on a monthly basis, which is great because it’s free. It only gets the 404s on Amazon (not the “currently unavailable”), but I just hand check my most popular pages. I always try to balance the value of my time with the value of the task.