Emacs

C-q <Control Character>

Example: Input escape character

C-q C-[

Shell, Vim

Ctrl+V <Control Character>

Example: Input escape character

Ctrl+V Ctrl+[

How to find a letter which should be pressed in combination with Control key?

<Control Character> in above explanation should be input in combination with Control key. The letter which should be input with Control key is a letter which is produced by setting 7th bit of the letter you want to input to 1, that is to say, by adding 0x40(64).
You can find it in `man ascii` etc.

Example:
Escape character is 0x1B(27).
Set 7th bit to 1, i.e. add 0x40(64) and you get 0x5B(91).
0x5B(91) is '['.
Therefore, to input escape character press Ctrl+[.

Line feed is 0x0A(10).
Set 7th bit to 1, i.e. add 0x40(64) and you get 0x4A(74).
0x4A(74) is 'J'.
Therefore, to input line feed press Ctrl+J.

Source: https://en.wikipedia.org/wiki/Control_character#How_control_characters_map_to_keyboards
エスケープ文字の入力方法 - nelnalog.note