Probably the single most important signal to look at and get the most information from in my opinion has to be the Oxygen sensor. It can tell you plenty about what's going on upstream once you learn how to interpret the signal. Here's a display sample of one showing a good waveform indicating proper switching.

The rise time (from lean to rich) is good, less than 100ms, and has good peak to peak range (above 800mv and below 150mv). The above sample was taken from a 92 Toyota EFI 4cyl Pickup with about 40k on the clock, engine at operating temp, above 2000 rpm. Not all sensor signals will look as clean as this example, they seem to vary based on application, and type of fuel induction.

Take a look at the test procedure that will verify the sensor using a labscope.

Go to this page and see some misc waveforms

Go to this page and see a simple lazy sensor.

Go to this page to see how certain conditions affect the waveform.