Verifying if a Function is a Radon-Nikodym Derivative for changing the numeraire

I am analyzing the following function within a financial mathematics framework: $f(t) = \dfrac{B(S; S) \cdot m(t)}{B(t; S) \cdot m(S)}$ where: $B(t; S) := \mathbb{E}_{t}^{\mathbb{P}} \left[\exp\left(-\int_{t}^{S}r_{f}(u)\, du\right)\right]$ and $m(t) := \exp\left(\int_{0}^{t} r_{f}(u)\, du\right)$ Definitions : $B(t; S)$ represents the price at time $t$ of a zero-coupon bond maturing at time $S>t$ , given the information available at time $t$ . $m(t)$ is a discount factor related to