You don't need to design your way out of a problem with a custom solution every time if you just add the scroll bar.
Being fooled into believing there's no more content is often incidental: some gap in the content aligns perfectly with the bottom of the window. (This is especially the case with designer websites which use a lot of blank spaces.) You shouldn't need a custom solution.
I wouldn’t consider having the website be designed to let you know that there’s more content a “custom solution”.
As for the rest of your comment. This really is a matter of preference. You’ve built a habit to look at the scroll bar. I’ve built a habit to try scrolling and see what happens.
But not knowing if you can scroll is caused by poor website design and not by a hidden scrollbar