After the post is saved (but hidden until the user solves ReCapcha) you can look at the database and analyse what's happening. Did ReCapcha turned away an interesting post by a legit user? What is the ratio of spans versus legit users? Etc.

If you turn both spammers and annoyed users with ReCapcha before any interaction,you will never know how many were legit and you won't be able to manually accept interesting legitimate content written by a user that is annoyed by Recapcha.