[Plugin: WP Super Cache] Statistics

You could count them but the act of counting them would add a further strain to your server.

If you really wanted to do it you could get two hit counters, or set up two goals in Google Analytics (I think) and add one to each place in the code where the corresponding “cached by” comment appears. I know this sounds vague, but look at the end of each blog page and you’ll see commented out text. Now look for that text in the code ..