= Solution
\b[Prefer molecular <rotational spectroscopy> or <rovibrational spectroscopy>], because nuclear <masses> enter their <reduced mass> directly. For an <isotopologue> with nuclear <reduced mass> $\mu_N$,
$$
E_{\rm rot}\propto\frac1{\mu_NR_e^2},\qquad
\omega_{\rm vib}\simeq\sqrt{\frac K{\mu_N}}.
$$
<Isotope> substitution therefore produces appreciable shifts and distinct sets of lines, assuming the same electronic potential and approximately unchanged <bond length> and <force> constant. A suitable heteronuclear molecule can provide electric-dipole rotational/vibrational spectra; other molecules may require <Raman spectroscopy>.
Atomic electronic energies instead depend on the electron-nucleus <reduced mass> $m_eM/(m_e+M)$, whose fractional change is suppressed by $m_e/M$. High-resolution atomic <spectroscopy> can still detect <isotope shifts>, including nuclear-size and many-electron <mass> effects; it is not intrinsically incapable of identifying <isotopes>. Molecular spectra generally offer the stronger <mass> sensitivity for this purpose.
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