You can monitor OBD communication with a second K-line/serial converter cable and
a connected terminal program, but you need to be able to set the correct communication speed.
And you will not be able to monitor the slowinit/fastinit patterns on the line.

Best way in my opinion is to use a logic analyzer and a voltage converter 12V->5V (two resistors are fine).
The Saleae Logic Analyzer is very good, the data collection unit just connects via USB to a regular PC,
so you can log a very long time with high resolution and the data analysis application can decode
serial data.

See here: http://nefariousmotorsports.com/forum/index.php?topic=568.msg4523#msg4523