In monolayer transition metal dichalcogenides, time-reversal symmetry—combined with broken space-inversion symmetry—defines the spin–valley degree of freedom. As such, the engineering and control of time-reversal symmetry by optical or magnetic fields constitutes the foundation of valleytronics. Here we propose a new approach for the detection of broken time-reversal symmetry and valley imbalance in monolayer WSe2 based on second-harmonic generation. At room temperature, our method can selectively probe a net valley imbalance generated by ultrafast, coherent and valley-exclusive optical Stark and Bloch–Siegert effects. This work demonstrates the potential and unique capabilities of nonlinear optics as a probe of broken time-reversal symmetry as well as a tool for ultrafast and non-destructive valleytronic operations.