First of all, simply looking at the data line and ignoring the clock line will tell you nothing.
does the clock run at constant baud rate?
Best bet is to connect the keyboard to a shift register and see what bytes come out with each keystroke (or hook it to an Arduino and use it as a data recorder).
I think there is even a PS/2 keyboard reading sketch or library available here. Check it out.
Yep, I tried that sketch, works fine with the Arduino, but with the scope I don't know what to do with the clock line, where to hook it up.
The PS/2 keyboard clock has a nominal frequency, but it's not critical (and it varies between keyboards). The concept of "baud rate" really doesn't apply here. The data is clocked out in sync with the clock line. When you detect the proper edge of the clock and then sample the data line, that is your zero or one. The actual speed ("baud rate") at which it comes out is irrelevant.
Also, the clock line doesn't constantly output a signal. It only sends a burst (one clock for each data bit) for every keystroke. And, each key has an "up" and "down" keycode. That's how you combine keys.
For example, if you hit CTRL, then C, the keyboard doesn't output a control-C, it outputs "Control key down, C key down" and it's up to you to determine that this combination means "ctrl-c".
The nice thing about this method is that virtually any key combination can be used. You can do "control-shift-alt-space-X" if you want to... just keep track of what key is up or down.
For more info, look here: http://www.computer-engineering.org/ps2protocol