There is currently no way in AuthManager to check if autocreation will be successful without actually doing it. Such a feature would be useful if we want to offer users an easy way to autocreate, but still bind it to user action to prevent T21161. Stackoverflow for example has (had?) this nifty feature where they do a check via JS when you first visit a site and then make a small overlay appear on the top of the page saying "It seems you are X, welcome back! Click here to be automatically logged in".