How un-clustered is a CLUSTER USING table in PostgreSQL
I am not entirely sure how much this information helps, but the system table pg_stats contains a correlation column:

select schemaname,tablename,attname,correlation 
from pg_stats 
where schemaname='public' 
order by correlation;


From the manual 


  Statistical correlation between physical row ordering and logical ordering of the column values. This ranges from -1 to +1. When the value is near -1 or +1, an index scan on the column will be estimated to be cheaper than when it is near zero, due to reduction of random access to the disk. (This column is null if the column data type does not have a &lt; operator.)


This value is on a per-column basis, so my guess is, that if the correlation for the first index column is near zero, the table is "un-clustered", if near -1 or 1 it is closer to the order of the index and thus "more clustered". This if course can only work for B-Tree indexes that are defined as ASC (I guess).