You have to send a GET request with Authorization header. 
Exemple with js :
req.open("GET", "https://id.twitch.tv/oauth2/validate", true);
req.setRequestHeader("Authorization", "OAuth " + hash_GET["access_token"]);
req.send(null);
You have to send a GET request with Authorization header. 
Exemple with js :
req.open("GET", "https://id.twitch.tv/oauth2/validate", true);
req.setRequestHeader("Authorization", "OAuth " + hash_GET["access_token"]);
req.send(null);