How to check for fragmenting on Cisco 2650 or Pix 6.3?

I have a Cisco 2650, which serves as default gateway on our LAN, connected to Pix 6.3.  There are a couple of other switches, a Cisco 3005 concentrator and Cisco 1841 Internet router - the last two are managed by AT&T.  On our WAN/VPN, which is also managed by AT&T, when remote locations ping a server in my office, only packets of up to 1472 (1500) go through.  If I change Cisco 2650's IP MTU to less than 1500, then larger packets get through.  Also, our WAN includes a VPN tunnel to a software company.  When our remote locations ping the software company's address, only packets of up to 1315 (1343) get through.  Changing Cisco 2650's IP MTU to less than 1315 limits the size of packets that are able to be transmitted as well.  For example, if I change Cisco 2650's IP MTU to 1000, only packets of up to 972 go through to the software company.

Both AT&T and the software company did the testing, and they are saying that one of my devices is failing to do proper fragmenting.  They are saying that Cisco 3005 (the concentrator) fragments packets properly.

How do I test for fragmenting on Cisco 2650 and/or Pix 6.3?  What are the exact commands?  Please explain for a newby.

Thank you.