Due to the equivalence of averaging over time and over ensemble, and the inherent time averaging during the act of measurement as a consequence of finite time resolution of the time measuring device, high frequency phenomena are indistinguishable from mixed states in standard quantum mechanics. In this talk we will analyze whether mixed states can be distinguished from quickly oscillating pure states under nonlinear evolution using weak measurements. We will show that if weak measurements in postselected systems are used, mixed states and quickly oscillating states produce different results. An experimental procedure is proposed which could in principle determine whether mixed states stemming from blackbody radiation, decoherence, thermalization in solid state materials, Unruh radiation and Hawking radiation, among others, are secretly quickly oscillating pure states under nonlinear evolution or fundamentally mixed. The analysis presented in this talk applies to all fast oscillations, including those at Planck scale. As such, tabletop weak measurements can be used to probe (very specific) potential high energy behavior, where strong nonpostselected measurements cannot be applied. This talk will also demonstrate that weak measurements are not equivalent to a set of strong measurements without postselection since measurements which are impossible for all practical purposes need to be excluded.