void OnCollisionEnter2D(Collision2D collision)
{
    ContactPoint2D contact = collision.GetContact(0);

    Vector2 hitDir = -contact.normal;
    hitDir.Normalize();

    float dotUp = Vector2.Dot(hitDir, Vector2.up);

    if (dotUp > 0f)
        //bas
    else
        //top
}
---------------------------------------------------
//trigger

void OnTriggerEnter2D(Collider2D other)
{
    Vector2 hitDir =
        ((Vector2)transform.position - other.ClosestPoint(transform.position)).normalized;

    float dotUp = Vector2.Dot(hitDir, Vector2.up);

    if (dotUp > 0f)
        //bas
    else
        //top
}