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.

https://sites.google.com/site/markgurries/dcc-welcome-page/advanced-topics/tools-for-dcc/dcc-packet-analysers

(note you may have to use the Internet Wayback Machine to use the links provided)

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