SQL & PL/SQL

Is there any way to find the sequence name for a particular column in table

ex

if empno in emp table uses the sequence empno_seq

so if a created that sequence i would know that empno_seq will be used by empno column.but other users how will they find the sequence name if they don't create one??

i tried some quires but unable to find

SELECT * FROM USER_SEQUENCES WHERE sequence_name = 'emp_seq'

what could be the "last_number" column in above query?? i could not understood what it refers??

Can any one help??