How can I see the AS number (not name)?

How can I collect NetFlow information only for a single interface without changing the settings on the router? I don't want to impact out existing Netflow server.

If I can't filter which interfaces I'm capturing NetFlow data for, how can I make a correlation between the interface and the AS that is sending the data?

Long story short, I'm trying to optimize my BGP routes to favor links with higher bandwidth, but I first need to see what ASNs are hogging all the bandwidth on my lower speed connections. Any suggestions would be greatly appreciated. Thanks