NOW is the best opportunity we will have. Teheran expects us to tread moderately with a measured response. israel has good cause to nuke Teheran, and that would certainly answer the test you speak of.
I wouldn't go quite that far just yet. But if Iran successfully tests a nuclear weapon, then we're in a critical situation and all kinds of very serious scenarios are possible. But given how relatively calm our world leaders seem to be these days, I think it's highly unlikely that Iran is close to testing a working nuclear weapon.
I tell ya though, I understand your desire to act now. It would solve some problems to blast them right now, but impatient action would cause even bigger problems for the long term. But I have a bad feeling that Iran is going to cause a lot of trouble for many months before their nuclear program is shut down one way or another.