In my opinion, one of the candidates to be a constant is IsGameWon, due to the fact that it can return only specified values: “true” or “false” and nothing else…
Another good candidate for a constant could be a “GetDifficultyLevel” - in case if game may be played in easy (0), normal (1) or hard (2) mode.
At the same time, IMO:
CheckPlayerGuess can’t be a constant because every time it may return different bulls and cows to the player, and it is impossible to say from the start, what number will it be …