I've had the opportunity to work on many internal projects, projects that either offline or protected by admin credentials, for different clients - I'm wondering what's the best way to refer to these projects on my LinkedIn/website/resume?
Just put it down. Write a simple description about it if the nature of the project is not obvious from the title.
If you're putting it in a website, than a few screenshots should sell the idea.
If it's for your resume, than screenshots are not necessary. Your potential employer will likely ask you more about it if he is interested. Companies also come and go; even if it was an online project, it can be shut down. As long as you can talk about it, you'll be fine.
For a job interview, you may also show a demo, using fake data.