IPhone : Check if variable is string or not in swift

I want to get this : if texfield isnt empty, and it isnt string, show ... and if it is show this...

I found a way how to check if it isnt empty, but I can't think of how to check if it string or not.

    if input != nil {

        var resultOutput = input! * 7

        result.text = "Your cat's age is \(resultOutput)"

    } else {

        result.text = "Please enter your cats age"

    }