Determine if Absolute or Relative URL

I have a relative or absolute url in a string. I first need to know whether it is absolute or relative. How do I do this? I then want to determine if the domain of the url is in an allow list.

Here is my allow list, as an example:

string[] Allowed =
{
   "google.com",
   "yahoo.com",
   "espn.com"
}

Once I know whether its relative or absolute, its fairly simple I think:

if (Url.IsAbsolute)
{
    if (!Url.Contains("://"))
        Url = "http://" + Url;

    return Allowed.Contains(new Uri(Url).Host);
}
else //Is Relative
{
    return true;
}