My mother-in-law had misas neshika. When my father-in-law saw her in the morning, she had a calm look on her face – there was no sign of any pain or trauma.
What does this meramez about the nifteres?
Rabbi Tzaddok Hacohen (of Lublim, Peri Tzaddik, Metzora 10) explains that misas neshikah occurs when a person merits the revelation of the Shechinah itself, and the soul simply leaves the body in clinging to the Shechinah. A similar concept is mentioned by Sefer Hamakneh (Kiddushin 39b), who writes that the righteous merit misas neshikah, which implies death at the hands of Hashem Himself (as described by the Torah of Moshe and Aharaon).
Of course, if your mother-in-law died in this way, it implies a very high spiritual level.
However, I am not sure of how easily a “misas neshikah” can be diagnosed. Either way, to pass away in such a calm and serene manner does indicate spiritual elevation.