What do you mean by "default" database? Do you mean the seed or starter database that is created during installation, or one that is the "default" because you have ORACLE_SID set as a user or system variable?
If you have ORACLE_SID set, at a DOS prompt, you can enter
"echo %ORACLE_SID%" and that will tell you what your "default" database is.
To see the names of instances that may be running, you can look at services (look for OracleServiceDBNAME, where DBNAME is the all-uppercase name of your SID, or database).