I am aware that a watched pot never boils, but sometimes I need to know how much time remains on a download. On a Mac, there are two ways to keep an eye on the progress of a download.
1. Launchpad in the Dock
For files being downloaded from the Mac App Store, the easiest way to check their progress is by mousing over the Launchpad icon in the Dock. You'll see a progress bar below the Launchpad icon and above the number of megabytes currently downloaded and the total file size.
Alternatively, since you already have the Mac App Store open when downloading a file from it, you can also keep an eye on downloads from within its confines. For new apps, you'll need to click through to an app's page. Below the blue Installing button is a progress bar that estimates how many minutes remain before the download is complete. When updating apps, you can see their progress on the Updates tab.
2. Downloads folder in the Dock
For files you download not from the Mac App Store but from the internet, the Launchpad offers no help. If you've dragged your Downloads folder to the Dock, then you can mouse over it to see a progress bar of a file currently being downloaded. Unlike the Launchpad, the Downloads folder doesn't show you the MB count when you mouse over its icon.
You can also check on a download's progress from the Downloads folder in Finder. Next to a file being downloaded, Finder shows a little clock-dial icon to show the download's progress.
In related news, check out 3 ways to install apps from unidentified developers on MacOS Sierra.