Pricom

Pricom made a very nice DCC protocol analyzer with an LCD screen.

http://www.pricom.com/Trains/DCCTester.shtml

Apparently no longer sold because of lack of the LCD screen.

You can update the firmware yourself, using a special cable:

Wiring for firmware update cable.

current and last firmware version: v1.52

Download page for the Pricom tester

If you find one, buy it.

NCE also makes a packet tester, see the NCE page.