Unused and invalid indexes in Oracle 8.1.7.4

My application runs very slow after I did a database import.
I doubt there is some problem in the indexes. I have the list of indexes used by my application with me.

What all checks I can perform ( in Oracle 8.1.7.4 ) to confirm that all my indexes have been created and working fine.

For some or (all) your queries, run the explain plan and see if the Indexes are being used. If not rebuild the index.
2) Try to analyze the tables.

If your application ran fine before and bad after
importing, calculate statistics may worth to check.