Once you have lights in your scene, you can set up your display options to display shadows in the viewport. This makes it easier to determine what shadows will look like at render time.
To display shadows in the viewport
Set up lights in your scene.
On the Shadow tab of the light node’s parameters, set Shadow type to either Depth map shadows or Ray-traced shadows (more information).
Turn on the Display shadows button in the display toolbar on the right side of the viewer.
Click the Viewport Options menu (the left menu in the top right corner of the viewport) and choose Display > Display Options, or press D to open the display options window.
Click the Misc tab and turn on the Shadows checkbox.
You can also turn on the Projected Textures checkbox which will clean up the look of your shadows.
Displaying shadows in the viewport may slow down Houdini, since it needs to generate a shadow map for each light casting shadows.