Massive (Milky Way-like) galaxies at high redshift sometimes show short-timescale variability of their star formation rate — the bursty regime of star formation. Bursty and steady modes of star formation can be distinguished based on comparison of star formation rate (SFR) indicators sensitive to different timescales, for example, increased Hα/UV ratios can be evidence of recent bursts of star formation in a galaxy. However this method gives only the current state of star formation in the galaxy and does not indicate anything about the length of the bursty mode or the time of transition to the steady regime. For 11 galaxies from the Feedback in Realistic Enviromnet (FIRE) simulations, we show that the scatter of the [O/Fe] distribution has a strong correlation with level of burstiness of star formation, and the observed age-scatter relation reflects dramatic changes in the evolution of the SFR. We also discuss the effect of various observational constraints and the potential applicability of the method to real Milky Way data.