How to identify overflow rows in DB2 LUW
You are likely correct to suspect compression as the cause of overflows. Having a clustered index by itself does not cause row movement outside of a reorg.

You can use the db2dart (Database Analysis and Reporting Tool) utility to dump formatted table pages; the "Record Type" field will identify which records have overflown. For example, run

db2dart yourdb /dd /tsi 2 /tn YOURTABLE /ps 0 /np 0 /v n


to dump all pages (/ps 0 /np 0) from the table YOURTABLE (note the uppercase name) in tablespace 2.

Overflown record will appear like this: 

        Slot 203:

           Offset Location = 1580  (x62C)
           Record Length = 12  (xC)

           Record Type = Pointer Record (tombstoned) (PUNC)

               Overflow Record Location:  Page 10  Slot 33


with their actual data in the new location:

        Slot 33:

           Offset Location = 3528  (xDC8)
           Record Length = 26  (x1A)

           OverFlow Record with Back Pointer

               Backpointer Location:  Page 7  Slot 203

           Compressed Record:  Row_Compressed

           Actual Record Length = 52  (x34)

           Record Type = Table Data Record (FIXEDVAR) (PUNC)

           Record Flags = 0


You can use the verbose flag (/v y) to include the actual row data in the report. For a large table you probably want to select a subset of pages by specifying a sensible value for /np. 

Don't forget to FLUSH BUFFERPOOL ALL before running db2dart on an active database, otherwise the data on disk, where the utility reads them, may not reflect changes still in the bufferpool.