How can I know whether MySQL, SQLite, PostgreSQL, etc. are installed on Mac OS X 10.7?
Try the Terminal command mdfind (string) | grep include

For example:

$ mdfind postgres | grep include
/usr/include/postgresql/server/postgres_fe.h
/usr/include/postgresql/server/postgres_ext.h
/usr/include/postgresql/server/postgres.h
/usr/include/postgresql/internal/postgres_fe.h
/usr/include/postgres_ext.h
...

$ mdfind sqlite | grep include
/usr/include/sqlite3ext.h
/usr/include/sqlite3.h
/usr/include/php/ext/sqlite3/libsqlite/sqlite3.h
/usr/include/php/ext/sqlite/libsqlite/src/sqlite.h
...


This is a good way to determine if there are header files installed for the respective database libraries you ask about.