SELECT mycol FROM mytab WHERE substr(mycol,1,1) = ' ';
SELECT mycol FROM mytab WHERE mycol LIKE '% ' OR c LIKE ' %';
or
SELECT mycol FROM mytab WHERE substr(mycol,1,1) = ' ' OR substr(mycol,-1,1) = ' ';
or
SELECT mycol FROM mytab WHERE regexp_like(mycol,'(^ | $)');
To catch any nonprintable, such as space, carriage return, newline, vertical tab, and form feed:
SELECT mycol FROM mytab WHERE regexp_like(mycol,'(^[:space:]|[:space:]$)');
NOTE: With Oracle 11, if there are concerns with more than one possible white-space character before or after a text value then the new REGEXP_LIKE functionality is far superior to checking for each possible character value with the techniques offered (e.g.) Also checking for leading/trailing tabs, newlines, etc.