...the common hydrogen isotope[protium] and
deuterium [the rarer and heavier hydrogen isotope] have different
radio-emission wavelengths.
Rasdio astronomers have used this fact for decades...
I'm not 100% sure, but I think common hydrogen [protium] has a natural radio-emission wavelength of 21.6 centimeters, and deuterium is [not totally sure of this] something like 18-19 centimeters.

ALSO...different isotopes of elements
can be differentiated by their subtle
differences in mass.
So it's possible to build an isotopic
detector that works by [electromagnetically] "sorting out" the different isotopes of high-speed atomic nuclei according to their differing inertial responses to the force generated by an electro-magnetic field.
Whether "science experiments" put onboard spacecraft have used this principle to detect different isotopes in space, I don't know.

Jacob