Is my oscilloscope working or not?
See that BNC connector on the left, labeled "Cal."? "Cal." is short for "Calibration". Stick the tip of your probe into that (thanks to Peter Bennet for this clarification). It provides a square wave signal that's intended for calibrating probes. If you see the signal, the scope is working. If you don't see it, the calibration output might not be working, or the scope itself might not be working.