Help: How to find the bad query

On our production db, recently, full table scan/index scan (this data is collected by SALVANT's dba tool--Diagnostic Center) increased dramatically, while hit ratio for DB buffer, Dict and library cache are all at 100%, could somebody tells me how I can find the faulty query or  queries that use a full table scan instead of a index scan? We checked that all the index status are valid, aparently some queries are  not using indexes they are supposed to use, but how to pinpoint them?

Thanks for you help.  

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