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.