Short of blood tests, there is no real way. Anecdotally, I start "feeling" like normal anywheres from 3-5 weeks. This time however on the USP stack, I have not ceased feeling normal. Lovin it.
One sign peeps like to use is how low the boys are hangin but things like HCG have shown me that they can be swingin low even while shut down. While it can be "an" indicator, it is just that, and only that. (an indicator that is) Libido is another not so sure fire method as many things outside of natty test can effect it. I judge somewhat by my general sense of well being knowing that low test makes me feel like poo poo.