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Low-power circuits are important for many applications, such as Internet of Things. Device variations and fluctuations are challenging their design. Random telegraph noise (RTN) is an important source of fluctuation. To verify a design by simulation, one needs assessing the impact of fluctuation in both driving current $\Delta {I}_{d}$ and threshold voltage $\Delta {V}_{\textsf {th}}$. Many early works, however, only measured RTN-induced $\Delta {I}_{d}$. $\Delta {V}_{\textsf {th}}$ was not directly measured because of two difficulties: its average value is low and it is highly dynamic. Early works often estimated $\Delta {V}_{\textsf {th}}$ from $\Delta {I}_{d}/{g}_{m}$ (${V}_{g} = {V}_{\textit {dd}}$), where ${g}_{m}$ is the transconductance, without giving its accuracy. The objective of this paper is to develop a new Trigger-When-Charged (TWC) technique for directly measuring the RTN-induced $\Delta {V}_{\textsf {th}}$. By triggering the measurement only when a trap is charged, measurement accuracy is substantially improved. It is found that there is a poor correlation between $\Delta {I}_{d}/{g}_{m}$ (${V}_{g} = {V}_{\textit {dd}}$) and the directly measured $\Delta {V}_{\textsf {th}}$ (${V}_{g} = {V}_{\textsf {th}}$). The former is twice of the latter on average. The origin for this difference is analyzed. For the first time, the TWC is applied to evaluate device-to-device variations of the directly measured RTN-induced $\Delta {V}_{\textsf {th}}$ without selecting devices. [ABSTRACT FROM AUTHOR]