This sounds like stating the obvious, but a number is square iff its square root is an integer. So if you have a calculator, use the square root function. if you don't have a calculator, you may be able to use inequalities. e.g. 1000 isn't square because 31^2 = 961 and 32^2 = 1024. You can also rule out some possibilities: a square must end in 0, 1, 4, 5, 6 or 9.