Depending on what it is, you need different tools.
If it's serial or USB (or 1394), you can get analysers that plug into your computer. I used a serial analyser at my last job (for debugging fax and telex apps, no less). We've got a
USB analyser here at empeg -- they're not cheap.
If it's network traffic, then
Ethereal (sniffing the glue that holds the Internet together) is your friend.
Most other things, you want a full-on logic analyser, from HP or similar. Like genixia says, ask your boss
very nicely.