At termination, the argument has some merit. God can't be found by logic alone, as logic merely is the rules of propositions so as to determine whether an argument is strong or not, which depends on the truth of the premises. "How can you know?" is not a strong argument and not one I think I have ever used, but the FSM and all that spiel seem to attack such an argument...
I think that for the existence of God to be known, that He would have to reveal Himself. How? Is He going to visit six billion people's residences bodily? No. Rather, He reveals Himself through written history and archaeology.
Now, the next, and often mimicked rapport of the atheist is that, in essence anyway, written history cannot be trusted -- especially if it lends credence to prophecies being fulfilled. Also, there are games of categorization and definition which are employed to label texts, such as the Bible, to an effect similar on other atheists as the Index was to Catholics at one time. Though bearing no punishment, other than, perhaps, humiliation for reading such-and-such type of material, this labeling of "sacred history" (and other euphemisms for "not worth my time") serves to prevent the reading of such texts.
But back to the main part, though FSM and IPU are, seemingly to me at least, going after an unsaid argument - or at least unsaid by the opponents - , they're right to say that the non-ability to dis-prove something is not a good way to prove something...
But there are more options out there:
Ability to prove something is a good way to prove something.
Ability to dis-prove something is not a good way to prove something.
Non-ability to prove something is not a good way to prove something.
Non-ability to dis-prove something is not a good way to prove something.
More than the above actual, since there are three negatives that can be switched, but anyway: why would attacking the non-argument of,
Non-ability to dis-prove something is a good way to prove something
with,
non-ability to dis-prove something is not a good way to prove something
be anything more than a verbal negation? And a negation of what I think is a strawman at that.