The following statement delivers all tables on which histograms are created (due to the restrictions described above, histograms are not taken into account in columns with only one distinct value):

SELECT OWNER, TABLE_NAME FROM DBA_TAB_COLUMNS
WHERE NUM_BUCKETS > 1
GROUP BY OWNER, TABLE_NAME
UNION
SELECT OWNER, TABLE_NAME FROM DBA_PART_COL_STATISTICS
WHERE NUM_BUCKETS > 1
GROUP BY OWNER, TABLE_NAME;

As of Oracle 10g, you can also use the following query:

SELECT OWNER, TABLE_NAME FROM DBA_TAB_COLUMNS
WHERE  HISTOGRAM != 'NONE'
GROUP BY OWNER, TABLE_NAME
UNION
SELECT OWNER, TABLE_NAME FROM DBA_PART_COL_STATISTICS
WHERE  HISTOGRAM != 'NONE'
GROUP BY OWNER, TABLE_NAME;