How un-clustered is a CLUSTER USING table in PostgreSQL

Question

I have some tables which benefit greatly from CLUSTER ON/CLUSTER USING in Postgres SQL:

# CLUSTER table USING index_name;
# ANALYZE VERBOSE table;
# CLUSTER VERBOSE;

A maintenance task periodically runs CLUSTER VERBOSE to keep things fresh. But is there a test I can run to see how fragmented the table is, prior to running CLUSTER VERBOSE? Maybe something like:

# CLUSTER ANALYZE
table 40000 records. 4000 observed clusters, 5000 potential clusters (20% fragmentation)

Note that I use CLUSTER so data accessed at the same time is "defragmented" into a small number of disk blocks. For example I have thousands of attributes that go with each page. a CLUSTER page_attribute USING page_id; puts all the attributes next to each other, greatly reducing disk load.