How do we know if a true type font has code points above 0xFFFF ?
There's an API (GetFontUnicodeRanges/GetGlyphIndices), but it doesn't go above 0xFFFF, as I suspect you know.
There are 2 obvious methods for finding out programatically:
.ttffile (the spec is open-ish)
This answer has a .NET/C# solution: Get supported characters of a font - in C#
If running Windows 7+, you can call DirectWrite's
IDWriteFontFace::GetGlyphIndices to get the nominal glyph id's from the cmap of given code points, or
IDWriteFontFace1::GetUnicodeRanges (either Win 8+ or Platform Update for Windows 7) if you just want to know all the ranges. GDI
GetGlyphIndices and Uniscribe
ScriptGetCmap supported only the basic multi-lingual plane.