Each character is drawn in the system's default font and compared, pixel by pixel, with the box drawn for a character no font has.
measuring...
0123456789abcdefghijknopqrstuvwxyzðøþđħĸłŧƀƃƈƌƒƕƙƚƛƞƣƥƨƪƫƭƴƶƹƺƻƾǂǃǝǥȡȥȴȵȶȷȸȹȼȿɀɂɇɉɋɍɏɐɒɓɔɖɗɘɚɛɜɝɞɟɠɢɤɥɦɧɨɫɬɭɮɰɱɲɳɴɵɶɷɸɹɺɻɼɽɾɿʀʁʂʄʅʆʇʈʉʊʌʍʎʐʓʔʕʖʗʘʙʚʛʜʝʞʟʠʡʢʤʥʧʨʩʬʭʮʯͱͳͷͼͽβδζθλμξπτχψωϗϙϛϝϟϡϻϼджлцчшщъыэюяђљњџѥѧѩѫѭѯѹѻѽѿҁҋҏґҕҗҙқҝҟҡңҥҧҩҫҭҳҵҷҹҿӄӆӈӎӷӻӽӿԃԅԇԉԋԏԓԕԗԙԟԡԣԥሀሁሂሃሄህሆሇለሉሊላሌልሎሏሐሑሒሓሔሕሖሗመሙሚማሜምሞሟሠሡሢሤሥሦሧረሩሪራሬርሮሯሰሱሲሳሴስሶሷሸሹሺሻሼሽሾሿቀቁቂቃቄቅቆቇቈቊቋቌቍቐቑቒቓቔቕቖቘቚቛቜቝበቡቢባቤብቦቧቨቩቪቫቬቭቮቯተቱቲታቴትቶቷቸቹቺቻቼችቾቿኀኁኂኃኄኅኆኇኈኊኋኌኍነኑኒናኔንኖኗኘኙኚኛኜኝኞኟአኡኢኣኤእኦኧከኩኪካኬክኮኯኰኲኳኴኵኸኹኺኻኼኽኾዀዂዃዄዅወዉዊዋዌውዎዏዑዒዓዔዕዖዘዙዚዛዜዝዞዟዠዡዢዣዤዥዦዧየዩዪያዬይዮዯደዱዲዳዴድዶዷዸዹዺዻዼዽዾዿጀጁጂጃጄጅጆጇገጉጊጋጌግጎጏጐጒጓጔጕጘጙጚጛጜጝጞጟጠጡጢጣጤጥጦጧጨጩጪጫጬጭጮጯጰጱጲጳጴጵጶጷጸጹጺጻጼጽጾጿፀፁፂፃፄፅፆፇፈፉፊፋፌፍፎፏፐፑፒፓፔፕፖፗፘፙፚᎀᎁᎂᎃᎄᎅᎆᎇᎈᎉᎊᎋᎌᎍᎎᎏᐁᐂᐃᐄᐅᐆᐇᐈᐉᐊᐋᐌᐍᐎᐏᐐᐑᐒᐓᐔᐕᐖᐗᐘᐙᐚᐛᐜᐝᐟᐡᐢᐣᐤᐥᐦᐨᐩᐪᐫᐬᐭᐮᐯᐰᐱᐲᐳᐴᐵᐶᐷᐸᐹᐺᐻᐼᐽᐾᐿᑁᑂᑃᑄᑅᑆᑇᑈᑉᑋᑍᑏᑐᑑᑒᑓᑔᑕᑖᑗᑘᑙᑚᑛᑜᑞᑟᑠᑡᑢᑣᑤᑥᑦᑧᑨᑩᑪᑫᑬᑭᑮᑰᑱᑳᑴᑵᑶᑷᑸᑹᑺᑻᑼᑽᑾᑿᒀᒁᒂᒃᒄᒅᒆᒇᒈᒉᒊᒋᒌᒍᒎᒏᒐᒑᒒᒓᒔᒕᒖᒗᒘᒙᒚᒛᒜᒝᒞᒟᒠᒡᒢᒣᒤᒥᒦᒧᒨᒩᒪᒫᒬᒭᒮᒯᒰᒱᒲᒳᒴᒵᒶᒷᒸᒹᒺᒻᒼᒽᒾᓀᓁᓂᓃᓄᓅᓆᓇᓈᓉᓊᓋᓌᓍᓎᓏᓐᓒᓓᓔᓕᓖᓗᓘᓙᓚᓛᓜᓝᓞᓟᓠᓡᓢᓣᓤᓥᓦᓧᓨᓩᓪᓫᓬᓭᓮᓯᓰᓱᓲᓳᓴᓵᓶᓷᓸᓹᓺᓻᓼᓽᓾᓿᔀᔁᔂᔃᔄᔅᔆᔇᔈᔉᔊᔌᔍᔎᔏᔐᔑᔒᔓᔔᔕᔖᔗᔘᔙᔚᔛᔜᔝᔞᔟᔠᔡᔢᔣᔤᔥᔦᔧᔨᔩᔪᔫᔬᔭᔮᔯᔰᔱᔲᔳᔴᔵᔶᔷᔸᔹᔺᔻᔼᔽᔾᔿᕂᕃᕄᕅᕆᕇᕈᕉᕊᕋᕌᕍᕎᕏᕑᕒᕓᕔᕕᕖᕗᕘᕙᕚᕛᕜᕝᕞᕟᕠᕡᕢᕣᕤᕥᕦᕧᕨᕩᕪᕫᕬᕭᕮᕯᕰᕱᕲᕳᕴᕵᕶᕸᕹᕺᕻᕼᕾᕿᖀᖁᖂᖃᖄᖅᖆᖇᖈᖉᖊᖋᖌᖍᖎᖏᖐᖑᖒᖓᖔᖗᖘᖙᖚᖛᖜᖝᖞᖟᖠᖡᖢᖣᖤᖥᖦᖧᖨᖩᖪᖫᖬᖭᖮᖰᖱᖲᖳᖴᖵᖶᖷᖸᖹᖺᖻᖼᖽᖾᖿᗀᗁᗂᗃᗄᗅᗆᗇᗈᗉᗊᗋᗌᗍᗎᗏᗐᗑᗒᗓᗔᗕᗖᗗᗘᗙᗚᗛᗜᗝᗞᗟᗠᗡᗢᗣᗤᗥᗦᗧᗨᗩᗫᗬᗭᗮᗯᗰᗱᗲᗳᗴᗵᗶᗷᗸᗹᗺᗻᗼᗽᗾᗿᘀᘁᘅᘆᘈᘉᘊᘋᘌᘍᘎᘏᘐᘑᘒᘓᘔᘕᘖᘗᘘᘙᘚᘛᘜᘝᘞᘟᘠᘡᘥᘦᘧᘨᘩᘪᘫᘬᘭᘮᘯᘰᘱᘲᘳᘶᘷᘸᘹᘺᘻᘼᘽᘾᘿᙀᙁᙂᙃᙄᙅᙆᙇᙈᙉᙊᙋᙌᙍᙎᙏᙐᙑᙒᙓᙔᙕᙖᙗᙘᙙᙚᙛᙜᙝᙞᙟᙠᙡᙢᙣᙤᙥᙦᙧᙨᙩᙪᙫᙬᙯᙰᙱᙲᙳᙴᙵᙶᙷᙸᙹᙺᙻᙼᙽᙾᙿᢰᢱᢲᢳᢴᢵᢶᢷᢸᢹᢺᢻᢼᢽᢾᢿᣀᣁᣂᣃᣄᣅᣆᣇᣈᣉᣊᣋᣌᣍᣎᣏᣐᣑᣒᣓᣔᣕᣖᣗᣘᣙᣚᣛᣜᣝᣞᣟᣠᣡᣢᣣᣤᣥᣦᣧᣨᣩᣪᣫᣬᣭᣮᣯᣰᣱᣲᣳᣴⶀⶁⶂⶃⶄⶅⶆⶇⶈⶉⶊⶋⶌⶍⶎⶏⶐⶑⶒⶓⶔⶕⶖⶠⶡⶢⶣⶤⶥⶦⶨⶩⶪⶫⶬⶭⶮⶰⶱⶲⶳⶴⶵⶶⶸⶹⶺⶻⶼⶽⶾⷀⷁⷂⷃⷄⷅⷆⷈⷉⷊⷋⷌⷍⷎⷐⷑⷒⷓⷔⷕⷖⷘⷙⷚⷛⷜⷝⷞ