How do you know if a number divisible by 3?

Add together all the digits until you get a single figure answer. If that's divisible by three, so's the original number. E.g. 4,239 broken down to 4 + 2 + 3 + 9 = 18; 1 + 8 = 9, which is a multiple of 3. The original number is therefore divisible by 3