From a quick search on Stack Overflow I saw people suggesting the following way of checking if a cookie exists:
HttpContext.Current.Response.Cookies
Sorry, not enough rep to add a comment, but from zmbq's answer:
Anyway, to see if a cookie exists, you can check Cookies.Get(string), this will not modify the cookie collection.
is maybe not fully correct, as Cookies.Get(string) will actually create a cookie with that name, if it does not already exist. However, as he said, you need to be looking at Request.Cookies, not Response.Cookies So, something like:
bool cookieExists = HttpContext.Current.Request.Cookies["cookie_name"] != null;