private GCheckout.EnvironmentType CheckAuthorization(string authHeader)
        {
            if (string.IsNullOrEmpty(authHeader))
            {
                Logger.Debug("No Authorization Header");
                return GCheckout.EnvironmentType.Unknown;
            }

            if (authHeader.Contains("Basic"))
                authHeader = authHeader.Substring(authHeader.IndexOf("Basic") + 5);

            var base64Auth = authHeader.Trim().Base64Decode();

            var userPass = base64Auth.Split(':');

            if (userPass.Length < 2)
            {
                Logger.Debug("Authorization Header was malformed.  Raw: " + authHeader + ", Base64Decoded: " + base64Auth);
                return GCheckout.EnvironmentType.Unknown;
            }
            if (userPass.Length > 2)
                userPass[1] = userPass.Skip(1).Join("");

            if (userPass[0] == Configuration.GoogleMerchantId &&
                userPass[1] == Configuration.GoogleMerchantKey)
            {
                return GCheckout.EnvironmentType.Production;
            }
            else if (userPass[0] == Configuration.GoogleTestMerchantId &&
                userPass[1] == Configuration.GoogleTestMerchantKey)
            {
                return GCheckout.EnvironmentType.Sandbox;
            }
            else
            {
                return GCheckout.EnvironmentType.Unknown;
            }
        }

Using GCheckout.dll

Join("") is an IEnumerable<string> extension I wrote (to concatenate strings with a separator)
Base64Decode() is a string extension I wrote (decode base64)

Configuration.Google[Test]Merchant{Id|Key} are just some static variables holding my info.

Basic Authentication (for Google Checkout)

google

C#