How do I check the download progress of Mountain Lion through the terminal?
Sure - the storeagent uses a folder deep within /private/var/folders to hold the download progress.

On my Mac it's downloading to this folder below - yours may change, but you should be able to search for the com.apple.appstore using mdfind and use du or ls to see the file size grow.

/private/var/folders/tv/xyw2rpln7hq4gw2m0prg_src0000gn/C/com.apple.appstore/497799835


The App Store has a nicer wrapper on the progress with a time estimate, but with bc and other tools, you could do the same in a short shell script exercise. 



If you can't locate your folder, this find command might be of use to narrow down your search....

sudo find /var/folders -type d -name com.apple.appstore -print