How to create method exist in a table

Follow this example

public static boolean exist(String255 _concatNum)
{
    boolean found;
    ;

    found = (select firstonly
                 RecId
            from
                 Thy_dimensionsNivels
             where
                 Thy_dimensionsNivels.ConcatNum == _concatNum).RecId != 0;

    return found;
}