We show that the transconductance of multilayer WSe₂ field-effect transistors serves as a direct electrical spectrometer of the intervalley relaxation time τ_(rm iv), previously accessible only by ultrafast optical techniques. Extending an equilibrium valley-thermodynamics framework with a single relaxation equation for the Γ-valley carrier fraction f_Γ(t), we predict three signatures: (i) a Lorentzian transconductance gₘ(ω)=g_(m,0)+g_(m,v)⁰/(1+iωτ_(rm iv)), whose imaginary part peaks at ω_c=τ_(rm iv)⁻¹ with opposite signs for bilayer and trilayer; (ii) a two-stage current transient after a gate step, exhibiting bilayer overshoot or trilayer undershoot; and (iii) sweep-rate-proportional hysteresis whose gate-voltage profile and the layer-dependent transconductance sign reversal distinguish valley from trap-induced dynamics. All three signatures provide quantitative electrical access to τ_(rm iv) with standard radio-frequency (rf) and direct-current (dc) instrumentation.