Fertilizer can explode just like a bomb or a missile. So just because there was an explosion doesn't mean it could only be a bomb or a missile.
But let us assume for a moment it was a powerful missile. Such ordnance would have been launched from somewhere by someone. Who? Nobody has claimed responsibility for launching a missile at the fertilizer fire. The government is not using the fertilizer explosion, missile or not, as a pretext to declare martial law. It is not being used as a pretext to declare war on some oil or opium rich country. The government is treating it like an accident.
I recall a bombing of a bar in Asia a few years back. The bar was frequented by Westerners. It put up a big mushroom cloud. A natural gas pipeline rupture added to the explosion. Some people claimed the bomb had been a "suitcase nuke." They said the glow of the cloud (which was a reflection of the fire on the ground) proved it. It wasn't a nuke. If it was a nuke everyone anywhere near would have been blinded. All of the electronics would have been fried, including the cameras that took the pictures of the glowing cloud.
The key is to look at how the government reacts. If it is using it as a pretext for something, then start looking for something wrong with the official story. Nobody in government is going to let a crisis go to waste, if they can help it.
Now, if West, Texas was going door to door searching houses for people hoarding Miracle Grow because of this fire, then I would give the missile theory more consideration. If Obama was accusing North Korea of attacking a Texas fertilizer plant fire, I would be suspicious. Otherwise, will only look at this as a reason not to live close to a fertilizer plant.
[F]orce can only settle questions of power, not of right. - Clyde N. Wilson