Check the manometer, the small U-shaped gauge on the pipe. If the liquid sits at different heights on each side, the fan is pulling. Level on both sides means the system is not running. Retest every two years regardless.
More detail
The manometer is the whole diagnostic, and it is designed so a homeowner can read it without tools. It is a clear U-shaped tube mounted on the vertical pipe, partly filled with colored liquid. When the fan is running and drawing suction from beneath the slab, the liquid is pushed higher on one side than the other, and the offset is the pressure the system is generating. Mark the normal offset with a pen when the system is new so you have a reference. Both columns level at the same height means there is no pressure difference, which means the fan is not moving air. The usual causes are a failed fan, a tripped breaker or unplugged fan on the circuit, or a disconnected or blocked pipe run. None of those are dangerous in themselves, but the house is unprotected until the fan runs again. A manometer showing normal offset is good evidence the system is pulling, and it is not proof the house is below 4.0 pCi/L. The gauge measures suction, not the air you breathe. That is why the standing recommendation is to retest every two years, after any significant foundation, basement, or HVAC work, and after any fan replacement. Test kits for this cost far less than a service call. Fans have a service life. Most inline radon fans run five to ten years of continuous duty before they fail, so a system installed with the house a decade ago is due for attention even if the gauge still reads correctly today. In Greater Columbus and Central Ohio, the common install is a basement system with the fan mounted outside the living envelope, so the manometer sits on the vertical pipe near the suction point. Across the mix of pre-war stock in the older Columbus neighborhoods, mid-century ranches in the inner suburbs, and large 1990s-onward subdivisions in the outer counties, older stone and block basements frequently carry two suction points, which means two gauges to read rather than one.