Correction: We will know that Iran is about to get the bomb when the Israeli cruise missile strikes set off a MASSIVE secondary explosion!
You forget that the US will actively warn Iran if such a plan is hatched by Israel and help shoot it down. After all we cannot allow our new ally to come to harm; further Iran nuking Israel will make both the mad mullahs and Obama happy - why else do you imagine Obama caved to all Iranian demands and the US never mentioned Iranian nuke development in North Korea ...
The only possible massive secondary explosions will come when the Iranian ICBMs strike NYC.