To check the signs of a bird

The Rambam’s words

To check the signs of fowl in order to differentiate between one which is kosher and one which is not kosher, as [Deuteronomy 14:11] states: "All birds which...."

לבדוק בסימני העוף עד שיבדיל בין טמא לטהור, שנאמר: "כל ציפור טהורה, תאכלו" (דברים יד,יא).

Who & when

“I have written all of the content of this commandment, its root, its laws and in which place it is practiced and at what time in the Order of Vayehi Bayom Hashmini (Sefer HaChinukh 153) concerning the examination of the signs of a [domesticated] beast, a [wild] animal, fish, and locusts — as the law of all of them is the same.”

Where in the Mishneh Torah

Hilchot Forbidden Foods הלכות מאכלות אסורות