1. In a natural language transcribed alphabetically, the frequency
distribution of neighbors of any letter are going to be determined by the
phonological properties of the letter itself. So, for example, the
frequency distribution of letters that come before and after 'A' will be
different from the frequency distribution of letters that come before and
after 'T', and both will be different from the frequency distribution of
letters in the text as a whole. 'T' will only rarely be followed by 'M',
for example, while that is much more common for 'A'.
2. Nulls have no phonological properties. If they are randomly larded
throughout a text, especially in high frequency, then the frequency
distributions of the letters that appear before and after them should be
more or less similar to the overall frequency distribution of those letters
in the text as a whole.
A variation on this test could be used to identify different ways of
representing the same letter, if you could correctly identify which
uniliteral/biliteral/triliteral combinations represented individual letters.
If you randomly alternate A with 8, for example, the frequency distribution
of neighboring letters should be nearly the same for the two allographs.