RF Emissions Testing and Discovery of Hidden Undocumented RF Device
There is no foolproof way but indeed it is possible in some cases with non-transmitting devices.  In fact that is how some "bug sweeping" devices called NLJD (Non-Linear Junction Detectors) work. The key is to sweep a frequency and look for faint return harmonics.  Any non-linear device will clip the waveform and cause a "retransmission" As long as you have a way or getting some energy into the device then you should get a response back. The non-linear device may be protection diode on the IC pads, transistor on power supplies and even voltage regulators.

It is very hard to do in some cases as the signal can get swamped/absorbed by capacitance.

If you have a perfectly shielded device then it becomes increasingly difficult. But shielding a device that has openings for sound is also hard to do.

Looking for faint frequencies from on board clocks is also possible, but there is usually a very large ambient level of electronic noise in the environment.