As soon as you accept a match you can see your opponents ranks. Additionally you can quickly access their opendota page to see your win+loss ratio vs them or to check their most picked heroes.

If you are looking for an easy install go to the releases tab on github there is a windows installer. If running from source a quick npm install and npm start.

Go to setup and locate the server_log.txt (its usually found at C:/Program Files (x86)/Steam/steamapps/common/dota 2 beta/game/dota/server_log.txt) Enjoy

Nope, what the app does is watch the server_log.txt file for changes. When you join a match that file gets updated with a list of the players in the match. I then use OpenDota's api to check each player's rank. No packet sniffing, no interaction with the Dota client at all.