Determining what AS' are being announced

I'm doing traffic analysis to try and determine the amount of traffic I'm sending to or through a particular AS. I'm wondering if there is a documented matrix or a utility that can determine what AS' a particular AS is announcing. Once a reasonably accurate as-path has been generated, I can tell flowscan to grab statistics for all the AS' in the path, but until I know that as-path, I can only get stats for say PSI, but not for it's customers. Thanx in advance.