How can I know which friends on Google+ are not using it? I have reached the limit in some circles, and want to delete the users who are in my circles but don't use Google+.
I recommend the Circloscope Premium for Chrome: 5 AUD for the extension and your circles management will be a piece of cake.
Circloscope premium lets you analyze people in your circles in numerous ways. For example, you can:
analyze the last post of all people in your circles (or a specific circle or mix of circles),
analyze your relationship with them (whether it's a two-way relationship, you're a follower, or they're your followers),
analyze their engagement with your posts (either by plussing, resharing or commenting on your posts),
analyze their relevance to you (based on how many interactions between you and them), etc.
I hope this helps, it sure has worked for me.