QGIS does not currently have a really good way to visualize point cloud data (i.e. as of 2018). Instead, I would recommend the excellent, open source plas.io web app. plas.io allows you to visualize the point cloud data with the ability to adjust particle size, point density, intensity, color, and a host of other features.
For example, the screenshot howsshows a point cloud visualization of Half Dome in Yosemite NP. I typically use the web app to visually scan the point cloud data to check for things such as scan line overlap and point density prior to processing.