There are ways, I would be very surprised if there are not certain people in Syria monitoring the situation and pushing it in one way or the other. No, I do not mean Illuminati, before someone starts on that.

I would not be surprised if western governments do, in fact, have a sample of the gas used, and have perhaps leaked information through back channels to certain international agencies what has happened.
Of course, without proof, and that proof cannot be used for fear of compromising agents, then all these agencies can do is 'speculate'.