How do you find out if a number is divisible by 3?

To determine if a number is divisible by 3, you can sum its digits and check if the result is divisible by 3. For example, for the number 123, you would add 1 + 2 + 3 to get 6, which is divisible by 3. If the sum is divisible by 3, then the original number is also divisible by 3.