Maybe you don't have the right display tool to show Unicode text, or maybe you need to see details that a standard character display can't show you, or maybe you just want to see the 4-hex-digit code point value for some wide characters. This will show you the 4-hex-digit details.
# assuming 5.8.1 or later:
perl -C9 -pe 's/([^[:ascii:]])/sprintf("\\x{%.4x}",ord $1)/eg' sometex
+t.utf8
# that is, leave ascii data as-is, convert wide characters to "\x{HHHH
+}"
If you leave off "-C9", you get to see the "underlying" utf8 byte sequences (in which case, a format of "\\x{%x}" would suffice), instead of the "official unicode reference" 4-hex-digit utf16 values. The "-C9" (same as "-CIi") causes STDIN and any input file handle to be read/interpreted as utf8 characters (i.e., works when reading from a pipe as well as from a file).
In fact, leaving off "-C9" makes it behave sort of like a typical hex-dump utility, except that only the bytes whose 8th bit is set will be converted to hex.
If your input is in some encoding other than utf8 and you want to see which unicode characters it will be turned into,
do it like this (using Cyrillic as an example):
perl -pe 'BEGIN{binmode STDIN,":encoding(iso-8859-5)"} s/([^[:ascii:]]
+)/sprintf("\\x{%.4x}",ord $1)/eg' < iso-cyrillic.txt