A number is divisible by 2 if its last digit is 0, 2, 4, 6, or 8. That is it.
Why this works: any number can be written as 10k + d, where d is the last digit. Since 10 is divisible by 2, the entire number is divisible by 2 if and only if d is.
Example: 1234 ends in 4 (even), so 1234 is divisible by 2. 1235 ends in 5 (odd), so it is not.