Finding out what characters a given font supports

Here is a method using the fontTools Python library (which you can install with something like pip install fonttools):

#!/usr/bin/env python
from itertools import chain
import sys

from fontTools.ttLib import TTFont
from fontTools.unicode import Unicode

with TTFont(
    sys.argv[1], 0, allowVID=0, ignoreDecompileErrors=True, fontNumber=-1
) as ttf:
    chars = chain.from_iterable(
        [y + (Unicode[y[0]],) for y in x.cmap.items()] for x in ttf["cmap"].tables
    )
    if len(sys.argv) == 2:  # print all code points
        for c in chars:
            print(c)
    elif len(sys.argv) >= 3:  # search code points / characters
        code_points = {c[0] for c in chars}
        for i in sys.argv[2:]:
            code_point = int(i)   # search code point
            #code_point = ord(i)  # search character
            print(Unicode[code_point])
            print(code_point in code_points)

The script takes as arguments the font path and optionally code points / characters to search for:

$ python checkfont.py /usr/share/fonts/**/DejaVuSans.ttf
(32, 'space', 'SPACE')
(33, 'exclam', 'EXCLAMATION MARK')
(34, 'quotedbl', 'QUOTATION MARK')
…

$ python checkfont.py /usr/share/fonts/**/DejaVuSans.ttf 65 12622  # a ㅎ
LATIN CAPITAL LETTER A
True
HANGUL LETTER HIEUH
False