I go to Google Analytics > Behavior > Site Content > Content Drill Down. At least I can see page views etc. for my custom pages.
I use a Category template with one or more custom content boxes for most of my custom pages and I can filter on category IDs in Analytics.
Obviously, if a custom page is tied to one of the standard quilts, pulling analytics is straightforward.
Several of our custom pages are completely standalone and that's where I'm having an issue.
I'll usually map a category node to my custom quilt (eg: CategoryPage.xyz), as @kgroneman suggests. It doesn't have to look anything like a CategoryPage quilt. It's usually pretty easy to switch standalone custom pages to custom CategoryPage quilts, though it would affect URLs, and you'd only be collecting analytics data moving forward.