The problem is that while this is trivial for Western languages I don't even know how you'd begin when presented with arbitrary text using an East Asian or African written languageNot actually that hard.
Consider a document which is encoded in either a) ASCII like you know it or b) ASCII where the top 4 bits and bottom 4 bits are transposed. How would you tell the difference? Well, one can imagine creating a histogram of the bits for each half of the bytes and comparing them to expectations based on the distribution of bits in naturally occurring English text. The half with most of the entries in the 0x5, 0x6, and 0x7 is the upper order half.
If you don't know what naturally occurring e.g. Japanese looks like in Unicode code points, take this on faith: flipping the order does not give you a document which looks probably correct. (Also, crucially, Japanese with the order flipped doesn't resemble any sensible document in any language -- you end up with Unicode code points from a mishmash of unrelated pages.)
P.S. Why care about that algorithm? Here's a hypothetical: you're a forensic investigator or system administrator who, given a hard drive which has been damaged, need to extract as much information as possible from it. The BOM is very possibly not in the same undamaged sector which you are reading right now, and it may be impossible to stitch the sectors without first reading the text. How would you determine a) whether an arbitrary stream of bytes was likely a textual document, b) what the encoding was, c) what endianness it was, if appropriate, and d) what human language it was written in?
Not actually that hard.
Consider a document which is encoded in either a) ASCII like you know it or b) ASCII where the top 4 bits and bottom 4 bits are transposed. How would you tell the difference? Well, one can imagine creating a histogram of the bits for each half of the bytes and comparing them to expectations based on the distribution of bits in naturally occurring English text. The half with most of the entries in the 0x5, 0x6, and 0x7 is the upper order half.
If you don't know what naturally occurring e.g. Japanese looks like in Unicode code points, take this on faith: flipping the order does not give you a document which looks probably correct. (Also, crucially, Japanese with the order flipped doesn't resemble any sensible document in any language -- you end up with Unicode code points from a mishmash of unrelated pages.)
P.S. Why care about that algorithm? Here's a hypothetical: you're a forensic investigator or system administrator who, given a hard drive which has been damaged, need to extract as much information as possible from it. The BOM is very possibly not in the same undamaged sector which you are reading right now, and it may be impossible to stitch the sectors without first reading the text. How would you determine a) whether an arbitrary stream of bytes was likely a textual document, b) what the encoding was, c) what endianness it was, if appropriate, and d) what human language it was written in?