Quick SQL question . . .

Can anyone tell me what is the easiest way for me to tell if a column is
unique or not?  I tried using DISTINCT ON and COUNT together in a SELECT
statement, but I can't seem to get the query to work:

SELECT DISTINCT ON (identno) count(identno) FROM some_table;

I was trying to figure out if the # of unique entries for a particular
column is equal to the # of total entries for that column.

Any suggestions?

Peter