How to find out if two numbers are relatively prime?

Swift 4 code for @williem-van-onsem answer;

func gcd(a: Int, b: Int) -> Int {
    var b = b
    var a = a
    var t: Int!

    while(b != 0){
        t = a;
        a = b;
        b = t%b;
    }
    return a
}

func relativelyPrime(a : Int, b: Int) -> Bool{
    return gcd(a: a, b: b) == 1
}

Usage;

print(relativelyPrime(a: 2, b: 4)) // false

Well in case they are relatively prime, the greatest common divider is one, because - if otherwise - both numbers could be devided by that number. So we only need an algorithm to calculate the greatest common divider, for instance Euclid's method:

private static int gcd(int a, int b) {
    int t;
    while(b != 0){
        t = a;
        a = b;
        b = t%b;
    }
    return a;
}

And then:

private static boolean relativelyPrime(int a, int b) {
    return gcd(a,b) == 1;
}

Euclid's algorithm works in O(log n) which thus is way faster than enumerating over all potential divisors which can be optimized to O(sqrt n).