1) A scroll bar
2) Clipped content
Many browsers hide the scroll bar, so the only thing left is 2).
For some reason designers think that it's a good idea to scale the content in such a way that the initial page load looks like a complete page.
This is so idiotic! You are actively misleading the customer! You are hiding all the marketing copy.
Some designers have realised this is a problem, and they add a little arrow to indicate you can scroll. Nobody understands those weird arrows except other designers.
The easiest way to tell users that there is more content is to show it! Make it so that part of the second headline is visible, and noone will miss the fact that there is more content below!
If you go for sexy screenshots, sure, go make your first section fill the browser window. If you want your visitors to read your page, don't do that!