Check if there are any object "behind" main object?

So I’m working on a new game, but I’m pretty new in unity (especially in c#), so I’m trying to figure out a way to solve this issue I have, and I’d like some help.

I have a 2D sprite (the player) that always faces the camera, and a 3D environment which is also made out of 2D sprites in different angels. You, as a player, can move the player, and also rotate the camera, using a controller.
In this game, you lose if you get out of the shapes. the thing is, Im not sure how to check for that scenario. how do I check if the player got out of the shapes? how can I check if there are no other objects behind it?

EXAMPLE
(sry for the link, the file upload server seems to be down) - so in that case, the player supposed to lose when he gets out of the red shape.

I thought about raycasting, but I’m not sure how does it work exactly, so If that’s a good idea let me know how to do that exactly.

Thank you in advance!

OK, I finally solved it using raycasting, and adding thin 3d colliders to the planes

    private void Update()
    {
        if(Physics.Raycast(player.transform.position, player.transform.forward, out hit, Mathf.Infinity, ~playerLayer) || Physics.Raycast(player.transform.position, -player.transform.forward, out hit, Mathf.Infinity, ~playerLayer))
        {
            //Still in the game
        }
        else
        {
            //Lose
        }

}