There are several basic tools one can use for this. I'd prefer to use the commandline rtl_sdr or rtl_fm from the rtl-sdr Debian package. The issue I ran into was that we're tuning into a relatively narrow-band signal, so the tuning is sensitive, and small tuning errors make you miss the signal you want entirely. RTL-SDR is a cheap device, and its tuning inaccuracy alone is enough to break this. There exists an RTL-SDR calibration tool to compensate for the hardware inaccuracy, but I still wasn't able to successfully tune into the frequencies, as defined in the LAPD channel list linked above. I didn't push on this very hard, so this could very well be my fault.

So instead of the commandline tools, I ended up GQRX. Pretty much all the LAPD frequencies are in the 484MHz range or the 506MHz range. I set the tuner into the right neighborhood, then the FFT waterfall plot in GQRX visually shows you which frequencies are active. You can roughly tune in simply by looking at the plot, and you can fine-tune by listening to the demodulated signal, trying to find the characteristic digital buzz and no static. There are multiple digital-sounding channels and multiple types of encoding are present (sound different). You can play around to find a signal that dsd knows how to decode. Note that since we're now looking for channels empirically, we compensate for tuning inaccuracies, but the LAPD frequency list becomes useless, and we don't even know what specifically we're listening to.

The GQRX window looks like this:

We're clearly listening to an active transmission: we're tuned to the channel indicated by the red line, and the waterfall plot shows intermittent activity there. The signal is intermittent because the transmitter is only active when there's data to send, i.e. when the human talking into the radio is pressing the button.