You might be able to detect a regenerative receiver. This is because this receiver topology uses positive feedback to increase the Q of the tuned circuit. If the feedback is too much it becomes an oscillator which transmits quite conspicuously.
Detecting just any huperhet receiver is a different matter. The coupling between the LO and the antenna is not deliberate as it is in a regenerative receiver.
That's not to say there aren't detectable emissions. Anything with an oscillator could be detected. That includes all digital electronics, and every switch mode power supply. In fact looking around me now the only electronic device I see which does not have an oscillator in it is a fish tank heater.
All these oscillators are detectable, if you know well enough what you're looking for and have enough resources. But detecting the emissions from a superhet receiver among all the noise of everything else with an oscillator in it is a challenge, to say the least.
If anything, SDRs are easier to detect. Some SDRs have at least one mixer which would have an LO. Some are direct sampling, meaning they ADC runs at such a high rate it doesn't need to down-convert the signal before sampling it. But all SDRs have an ADC, and an ADC requires a clock, which is another oscillator. Whatever is processing the digital data will undoubtedly have another clock and a plethora of other noisemakers.