Technically, it would be impossible to truly know without "computing" at all, but once you add 2 and 3, you would be able to know that the sum of all three numbers of positive without accounting for the -2, because 6 is a greater absolute value than -1.
Some examples:
12 + - 4
7 + - 3
18 + -10
All will be positive because those positive numbers (12, 7, 18) are greater than the others (4, 3, 10).