Use the divisibility rules (this applies to all integers)
For 2: a number is divisible by 2 if it ends in an even integer (10, 22, 34, 46, 58 all work)
For 3: a number is divisible by 3 if the sum of its integers is divisible by 3 (for instance, 15, 33, 27, etc.). To prove this we need to use modular arithmetic, which is a more advanced concept (high school), so just remember it for now
For 6: a number is divisible by 6 if it's divisible by both 2 and 3
For 78, 8 is an even number, so it is divisible by 2.
7 + 8 = 15, so it is divisible by 3
And because it is divisible by 2 and 3, it is also divisible by 6.
Alternatively, you could just quickly divide it by 2, 3, or 6 to see if there is a remainder
Hope this helps :D