This page aims to document the Unicode coverage of all the fonts (including non-free ones) in Debian. It was created using fontforge, font-manager and gucharmap. You should also read the DebianInstaller/GUIFonts page to find out about the fonts used by the installer.
Please add any fonts which might fill in some of these blocks to the Fonts/Missing page.
Quick test
Iceweasel seems to render all Wikipedia languages:
http://meta.wikimedia.org/wiki/List_of_Wikipedias
http://git.wikimedia.org/blob/mediawiki/core.git/HEAD/languages/Names.php
Incomplete blocks
Ancient Greek numbers: U+1018B and U+1018C only available in Unifont Upper
Arabic mathematical alphabetic symbols: U+1EEF0 and U+1EEF1 missing
Armenian: U+058D and U+058E only available in Unifont
Bengali: U+0980 only available in Unifont
Combining diacritical marks supplement: U+1DE7 to U+1DF5 only available in Unifont
Combining half marks: U+FE27 to U+FE2D missing
Cuneiform: U+1236F to U+12398 missing
Cuneiform numbers and punctuation: U+12463 to U+1246E and U+12474 missing
Currency symbols: U+20BB only available in Unifont
Cyrillic extended B: U+A674 to U+A67B and U+A69F only available in Unifont
Cyrillic supplement: U+0528 to U+052F only available in Unifont
Dingbats: U+2700 only available in Unifont
Enclosed alphanumeric supplement: U+1F10B and U+1F10C only available in Unifont Upper
Georgian: U+10C7, U+10CD and U+10FD to U+10FF only available in Unifont
Georgian supplement: U+2D27 and U+2D2D only available in Unifont
Kannada: U+0C81 only available in Unifont
Latin extended-D: U+A794, U+A796, U+A797, U+A7AB to U+A7AD, U+A7B0, U+A7B1 and U+A7F7 only available in Unifont
Latin extended-E U+AB3D, U+AB3E, U+AB43 to U+AB47, U+AB49 to U+AB4C, U+AB53 to U+AB5B and U+AB5F only available in Unifont
Limbu: U+191D to U+191E only available in Unifont
Malayalam: U+0D01 only available in Unifont
Miscellaneous symbols and arrows: U+2B4D to U+2B4F, U+2B5A, U+2B5B, U+2B5E, U+2B5F, U+2B6E, U+2B6F, U+2B88 to U+2B95, U+2B98 to U+2B9F, U+2BA8 to U+2BB9, U+2BBF to U+2BC8 and U+2BCC to U+2BD1 only available in Unifont
Miscellaneous symbols and pictographs: U+1F321 to U+1F32C, U+1F336, U+1F37D, U+1F394 to U+1F39F, U+1F3C5, U+1F3CB to U+1F3CE, U+1F3D4 to U+1F3DF, U+1F3F1 to U+1F3F7, U+1F43F, U+1F4F8, U+1F4FD, U+1F4FE, U+1F53E, U+1F53F, U+1F540 to U+1F548, U+1F54A, U+1F568 to U+1F579, U+1F57B to U+1F5A3 and U+1F5A5 to U+1F5FA missing (some available in Unifont Upper)
Myanmar extended-A: U+AA7C to U+AA7F only available in Unifont
Ornamental dingbats U+1F650 to U+1F671, U+1F673, U+1F674 and U+1F676 to U+1F67F missing
Playing cards: U+1F0E0 to U+1F0F5 only available in Unifont
Runic: U+16F1, U+16F2, U+16F5, U+16F7 and U+16F8 only available in Unifont
Sinhala: U+0DE6 to U+0DEF only available in Unifont
Supplemental punctuation: U+2E3C, U+2E3E and U+2E3F only available in Unifont
Telugu: U+0C00 and U+0C34 only available in Unifont
Transport and map symbols: U+1F6C6 and U+1F6C8 to U+1F6F3 missing
Vedic extensions U+1CF8 to U+1CF9 only available in Unifont
Completely empty blocks
Some blocks have bitmap glyphs available in Unifont and Unifont Upper, noted with an asterisk (*) below.
Bassa Vah (U+16AD0...)
Caucasian Albanian (U+10530...)
Chakma (U+11100...)
Combining diacritical marks extended (U+1AB0...) *
Coptic epact numbers (U+102E0...) *
Duployan (U+1BC00...)
Elbasan (U+10500...)
Geometric shapes extended (U+1F780...)
Grantha (U+11300...)
Khojki (U+11200...)
Khudawadi (U+112B0...)
Linear A (U+10600...)
Mahajani (U+11150...)
Manichaean (U+10AC0...)
Mende Kikakui (U+1E800...)
Meroitic cursive (U+109A0...) *
Meroitic hieroglyphs (U+10980...) *
Miao (U+16F00...)
Modi (U+11600...)
Mro (U+16A40...)
Myanmar extended-B (U+A9E0...) *
Nabataean (U+10880...)
Old North Arabian (U+10A80...) *
Old Permic (U+10350...)
Pahawh Hmong (U+16B00...) *
Palmyrene (U+10860...)
Pau Cin Hau (U+11AC0...)
Psalter Pahlavi (U+10B80...) *
Sharada (U+11180...)
Siddham (U+11580...)
Sinhala archaic numbers (U+111E0...)
Sora Sompeng (U+110D0...)
Supplemental arrows-C (U+1F800...)
Takri (U+11680...)
Tirhuta (U+11480...)
Warang Citi (U+118A0...)
Some conscripts in the ConScript Unicode Registry (such as Klingon, Tengwar and Visible speech) have bitmap glyphs available in Unifont CSUR and Unifont Upper CSUR.