Detecing a Number

I have a repeating function to add. The user enters the number of how many numbers they want to add, then it repeats a form field. What I want to do is detect to make sure that the value they enter is a number. If not, don't do the do while loop and if it is, then go ahead and do it. How can I do this? Thanks, Stephen Craton http://www.melchior.us "What is a dreamer that cannot persevere?" -- http://www.melchior.us