2. Using It: Fonts & Encodings
i) Arial, Times New Roman, Calibri, Verdana, Helvetica, and Courier all support the upside down (aka inverted) question mark. In fact, practically all fonts you are likely to encounter will support it. It is technically possible for a font not to include a glyph for the upside down question mark, but it's such a common character that you generally do not have to worry about font support.
ii) The upside down question mark is included in the Latin-1 encoded standard, the Windows-1252 encoding standard, and of course the Unicode character set (U+00BF). Not all encodings include the upside down question mark, but you are unlikely to encounter a program or website that defaults to an encoding that does not support it. In other words, as with fonts, you generally do not have to worry about encodings or character sets.
iii) If you want to be certain that the upside down question mark will appropriatly display, then test it!