Tell how you know your answer is correct?

It depends if you needed to divide to find the answer you multiply to check if the answer is correct.
I other words use inverse operation.

You could use the inverse operation. Is it is an addition problem, then you can subtract to check your answer. For example the problem 5+4=9 can be checked by solving the equations 9-4=5 and 9-5=4. The same thing goes for multiplication and division, if it is a multiplication problem, then you can divide to check your answer and if it is a division problem, than you can multiply to check your answer.
I hope that helped! :-)

Hope i can you!