You might want to provide more information about your question.
- Is this attribute a database table column ? Do you want to make this check when rows are inserted ?
- Is this a local variable, or a procedure parameter ? (you tagged your question ‘PL/SQL Variables’)
A simlpe answer would be:
if >= 0 then
The other way is with SQL statement and CUSTOM OPERATOR:
Information how to create CUSTOM operator is HERE.