If you suspect power line ripple, you can set up a signal where you see some noise and then? switch triggering to "line".?? If the noise is mostly stable, it is power line noise (probably 120Hz versus 60Hz, but close enough).
Once you get the screw driver out:? You can also measure ripple without another scope using the AC scale of a meter.? You will not get an accurate to the mv measurement (unless it is a true RMS meter and you do some math to make it into P-P), but it will tell you if a large ripple is present.
A final hint, make sure you turn off any florescent lights near your bench before trying to track down noise.? This has bit me a few times.
