Used Hameg scopes are common and fairly inexpensive, as they were used a lot in schools (at least in my area), and their manuals are very good for learning about oscilloscopes.

To hook up your 1/4" signal, add a splitter to your output, and use one side to drive whatever you're outputting to, and the other side to measure the signal on the oscilloscope. Ready made 1/4" to BNC adapters are available, so you don't even have to use oscilloscope probes