Go over to the Diesel forum and read the sticky at the top first...
Then read the ' Am I low on R12 ' thread going on right now... I have answered the sight glass question over and over again...
You need a set of gauges if you are going to do your own stuff safely and expect your system to last for a while...
http://www.peachparts.com/shopforum/showthread.php?t=301351