Slow response is usually caused by a lazy or old O2 sensor. It can also come from replacing the OEM sensor with a junk parts store brand.
However, you asked how to ACTUALLY diagnose it. To do this, you need to verify that the signal is slow by looking at graphed data compared to a known good (Bank 2 sensor 1) on an OBD2 Scan tool while making sure the vehicle is in closed loop and RPM is held steady around 2k. If the switching from lean to rich is in fact slow, you should unplug the sensor, and manually drive the sensor lean and rich with an incandescent test light while watching for the manual input on the graphed data for the offending O2 sensor, or, by introducing a fuel source into the intake like propane, brake parts, etc, watching the signals go rich, and then introducing a vacuum leak and watching them drive lean. A few different ways to skin that cat.
All that said.... NTK O2's are cheap. Probably time to replace all 4 anyways.