How to find if a number is a Fibonacci

A positive integer ω is a Fibonacci number:

If and only if 5 * ω^2 + 4 OR 5 * ω^2 - 4 is a perfect square.

First lets write a method to test if a positive number is perfect square or not (note that there're many implementations of this method):

def is_perfect_square? pos_number
  Math.sqrt(pos_number).to_i == Math.sqrt(pos_number)
end

And here's the method of testing whether the number is a Fibonacci or not:

def is_feb? n
  is_perfect_square?(5 * n**2 + 4) ||
    is_perfect_square?(5 * n**2 - 4)
end