How to find speed in certain direction?

For signed world speed, just do:

  var speed = rigidbody2D.velocity.x;

For unsigned world speed, just do:

  var speed = Mathf.Abs(rigidbody2D.velocity.x);

If you are looking for local speed, then you will need to transform into local space:

  var dir = transform.InverseTransformDirection(rigidbody2D.velocity.magnitude);
  speed = dir.x;

...the results will be the speed in the local 'x' direction.

Edit: rereading your question. For only to the left, do:

     var speed = rigidbody2D.velocity.x;
     if (speed > 0.0) {
          speed = 0.0;
     }
     else {
          speed = -speed;
     }

This is will give you a positive value for any left movement and 0.0 for any right movement.