Regarding a Jewish servant, the Torah commands us, "You shall not subjugate him with hard labor; you shall fear Hashem" (Vayikra 25, 43). Rashi explains that one should not make his servant work needlessly. "Do not tell him, 'Warm up this drink for me' if he does not intend on drinking it. [Do not tell him], 'Hoe under this vine until I arrive' [just to keep him busy]. You might think to yourself, 'No one will know whether I need the work done or not.' Therefore it says, 'You shall fear [Hashem].'"
This is an idea which Rashi mentions numerous times throughout the parashios of Kedoshim and Emor: The litmus test for true yiras Shamayim is the way one acts when there is no one around to observe and scrutinize his behavior, and the Creator is the only One Who will know what he has done or his intention behind the action.
We are often exhorted to live our lives with a focus on penimiyus. How does one go about doing this? The answer, says Rav Wolbe (Shiurei Chumash), is by beginning to focus on those mitzvos and actions that are unbeknownst to anyone except Hashem. These actions are free from all external pressure, and therefore, they force a person to define for himself who he is, and consequently, how he will behave.
While this is certainly true on a personal level, it is also imperative to internalize this idea for the benefit of our interpersonal relationships. We often ascertain a person's level of Yiras Shamayim by their mode of dress or the shul where they daven. However, truthfully, these superficial trappings are not a valid barometer of one's yiras Shamayim. We only have the capacity to ensure that we ourselves fear Hashem, and act in accordance with His will regardless of whether or not we are being observed by other people.