DESCRIBE Table_Name;
OR
DESC Table_Name;
SELECT * FROM USER_TAB_COLUMNS WHERE COLUMN_NAME = 'MY_COLUMN'; -- Connected user
SELECT * FROM ALL_TAB_COLUMNS WHERE COLUMN_NAME = 'MY_COLUMN'; -- Other schemas
SELECT * FROM DBA_TAB_COLUMNS WHERE COLUMN_NAME = 'MY_COLUMN'; -- All tables
SELECT table_name, column_name, data_type, data_length
FROM USER_TAB_COLUMNS
WHERE table_name = 'MYTABLE'