It happens extremely rarely at the moment. You'd have to be going after a card the second it gets listed and competing against the bot-script primarily. Usually they just tie eachother. haha
So there's not too much to worry about.
And as soon as the bot script implements the lock system i'm sure matt can even turn on the enforcement earlier i'm guessing.
However to answer your question.
If you ever see either of these two things with a card that you tried to get (be it on yours or someone else who tried to get the same card)
Then go here: https://steemmonsters.com/cards/find?ids=
put in the Card ID after the =
That will show you who actually owns the card now.
Bam, much appreciated as always @jarvie