With any performance issue, the first step is to look at the code and understand the process. Given table structure/size/indexes, an experienced candidate should be able to suggest whether the process in the code is appropriate, and where any performance bottlenecks might be expected.
Look at the V$ tables to identify any poor performing SQL (either a single SQL that takes a long time, or one that happens too often).
If there's no problem in the SQL, then you can identify the location of PL/SQL problems using DBMS_PROFILER.
But I think that misses the point. If you go for a job, your resume/CV should indicate your level of experience. If I was interviewing someone with no Oracle experience, I wouldn't be asking them about how to identify performance issues.
While DBMS_PROFILER doesn't get used as much as it should (for some reason the package is not installed by default until 10G), I would expect anyone who has worked in Oracle for a year or more to be at least AWARE of tkprof.
If your resume/CV indicates that you have experience that you don't actually have, then you are trying to lie to your prospective employer. If you have straight from college/university/training courses and that is what the employer is expecting and looking for, then being honest about what you have learnt should not be a problem.