Nowadays everything that is text should be encoded as utf-8 without bom in files unless you have a very good reason not to.
That said, let's explore the problems if it is not
Here are a few examples:
|Encoding||Interpreted as||encoded text||Looks like||Python3|
|iso_8859-1||utf-8||é (0xE9)||error / �||"é".encode("iso_8859-1").decode("utf-8")|
|utf-8||ascii||é||error / �||"é".encode("utf-8").decode("ascii")|
If you see Â, Ã, Ä or Å followed by another character, you are viewing a file that is utf-8 encoded and contains C1 Controls and Latin-1 Supplement or Latin Extended-A characters in a viewer that interprets them as iso_8859 (ISO Latin).
If you see □, then your font does not contain the character. It may also be that the character is unprintable, like the vertical space that microsoft word adds incorrectly when pressing ctrl-enter.