I understand that a unicode string is a set of symbols and that utf-8 is a way of saving these symbols as a a set of bytes

Correct.

I understand also that "use utf8;" tells perl to interpret the perl file being read as being encoded using utf8 and containing unicode symbols.

Correct.

If I call a function, how do I know if it returns a unicode string or not.

The real question is "is the input a string of bytes or a string of characters, and is the output a string of bytes of a string of characters". Try the various combinations.

How is a unicode string encoded (utf8 or 16 etc?)

Earlier, you defined "unicode string" to mean a decoded string, so your question makes no sense.

If I have a unicode string, how do I output it to my console window so that it appears correctly?

It's easiest with:

use open ':std', ':locale';
[download]

That does the equivalent of

my $enc = ...[ get encoding from locale ]...;
binmode STDIN,  ":encoding($enc)";
binmode STDOUT, ":encoding($enc)";
binmode STDERR, ":encoding($enc)";
[download]