If you haven't spotted her until the loading screen you are clearly doing something wrong.
Here is how is an example of how I spot a Riven main before the loading screen:
NoobKiller#REKT69:"top or feed"
Now, although we do know she is probably a Riven main, we should make sure we are not mistaking her with a Vayne mayne, in this case I recommend asking this fallowing question: "what's your opinion about 1 V 5'ing the enemy team", if she says: "I will kill these scrubs EZ", then she is a Vayne Main, but, if she replies with this following statement:"omg reported I afk"
You know she is a Riven main.