Every expression have some value after the evavulation. If the resultant value is  neither TRUE nor FALSE then it is known as a boolean expression, else if come any other values then it is know as normal expression. So we can find an expression as boolean or normal by it resultant value...
  for example   :
   a++; (where a=10) the expression' svalue becomes 10. (NORMAL)
                           
 a>10; this expression,s value becomes TRUE or FALSE, depending  upon the value of a. so it is boolean.