How to test for a number

[Kevin J]

How do you make an If check to check to see if a user has entered a
number and not a letter?
I can stop it from doing a divide by 0 doing this:

on mouseUp
put field"num1" into tAdd
put field"num2" into tAdd2
  if tAdd2 is 0 then
    answer "You can't divide by 0"
  else
    put tAdd/tAdd2 into field"sum"
    end if
end mouseUp

Thanks

Kevin