Some truths are [*self-evident*](http://www.merriam-webster.com/dictionary/self-evident) or [prima facie](http://www.merriam-webster.com/dictionary/prima+facie), but many truths are *not self-evident*. It might be obvious in *hindsight* and that is frequently described as 20/20 (or perfect vision). Further, the truth might be said to be *obscured*, *masked* or *hidden* depending on context. 

Finally, Merriam-Webster gives these antonyms to *self-evident* -

>    arguable, contestable, debatable, disputable, doubtable, dubious, moot, problematic (also problematical), questionable

The best phrase I can think of is still *obvious in hind-sight*. Such a truth is unarguable once understood, but clearly not *self-evident*.