How to find the game lag before going into the game (mainly in League of Legends view) 6

How to find the game lag before going into the game (mainly in League of Legends view)
Several players asked about the $subject from me. Because some time lag is too high, you cannot play. You only get to know that from the starting (loading screen). There is a way to find it before you start play for windows users (there must be a way in other operating systems but I am ...