Interesting question, I have tried for years and until I visited the SC Putter Studio I could not tell exactly since I believed I needed more loft when in reality I needed less loft once I saw the high speed video of how my ball spins coming off my putter with my stroke.  I am now at 3 degrees loft when before I was at 5.  It reduced the amount of backspin however if I were to try to self diagnose I would have someone look at my impact position and was the face of my putter straight at impact or basically with little or no loft.  Just a guess, my serious recommendation is get under some high speed slow motion video to see what is really happening.. 

Good Luck!

Just to add to this, it may help to have a bold line marked around the whole ball when you try to record to see how the ball spins. If the ball shows back spin or is two much off the ground right after impact, then you may have too much loft. If the ball bounces or skids, you may have too little loft. The optimal loft will have forward spin very soon after impact. Check out this video at about 12:25 to see what I mean.

http://www.golfchannel.com/media/golf-fix-putting-march-1-2010/

To give credit where credit is due, here is a link to the 3 year old discussion where I found the video.

http://www.titleist.com/teamtitleist/team-titleist/f/7/t/14755.aspx