Explain how you know whether a sum is positive, negative, or zero without actually adding

When you're summing two numbers . . .

-- The sum is positive if the bigger one is positive.
(Then it doesn't matter what the sign of the other one is.)

-- The sum is negative if the bigger one is negative.
(Then it doesn't matter what the sign of the other one is.)

-- The sum is zero if both numbers are zero,
or
   the numbers have the same absolute value but different signs.