how to tell if oracle database is installed on pc

If it is an Oracle Client, it doesn't have bin/dbca

Oracle Home may indicate the installation's type. The default Oracle Home folder names are:

• Oracle Database: dbhome_1
• Oracle Client: client_1

Although to make sure, you should check the services looking for OracleServiceSID entry on Windows.

Run Command sqlplus on command prompt.

Check services of windows to know that the oracle database was installed on laptop or not.

Try to find oracle universal installer in all programs by this also you can get which version of database is installed.

Your second question is about default username/password For this you can give username : "/ as sysdba". It will not ask for password because it will authenticate thorugh OS authentication.

after login you can able to create new schema or user and tables.