  Odd-even harmonics emitted from a laser-target system imprint rich, subtle
information characterizing the system's dynamical asymmetry, which is desirable
to decipher. In this Letter, we discover a simple universal relation between
the odd-even harmonics and the asymmetry of the THz-assisted laser-atomic
system -- atoms in a fundamental mid-IR laser pulse combined with a THz laser.
First, we demonstrate numerically and then analytically formulize the harmonic
even-to-odd ratio as a function of the THz electric field, the source of the
system's asymmetry. Notably, we suggest a scaling that makes the obtained rule
universal, independent of the parameters of both the fundamental pulse and
atomic target. This universality facilitates us to propose a general pump-probe
scheme for THz waveform sampling from the even-to-odd ratio, measurable within
a conventional compact setup.
