public Transform startT = null;

public Transform endT = null;

public Vector3 boundsSize = Vector3.one * 3;

private Bounds bounds = new Bounds(Vector3.zero, Vector3.one * 3);

private void OnDrawGizmos()
{
    // Resize bounds
    bounds.size = boundsSize;

    // Transform line segment from world space to local space
    Matrix4x4 w2l = transform.worldToLocalMatrix;
    Vector3 startP = w2l.MultiplyPoint(startT.position);
    Vector3 endP = w2l.MultiplyPoint(endT.position);

    // Draw bounds
    Gizmos.color = Color.yellow;
    Gizmos.matrix = transform.localToWorldMatrix;
    Gizmos.DrawWireCube(bounds.center, bounds.size);

    // Draw line segment
    Gizmos.color = Color.cyan;
    Gizmos.DrawCube(startP, Vector3.one * 0.3f);
    Gizmos.DrawCube(endP, Vector3.one * 0.3f);
    Gizmos.DrawLine(startP, endP);

    Vector3 min = bounds.min;
    Vector3 max = bounds.max;

    Vector3 dir = endP - startP;
    Vector3 oneOverDir = new Vector3(1.0f / dir.x, 1.0f / dir.y, 1.0f / dir.z);

    // Slabs
    float _minSlabX = (min.x - startP.x) * oneOverDir.x;
    float _minSlabY = (min.y - startP.y) * oneOverDir.y;
    float _minSlabZ = (min.z - startP.z) * oneOverDir.z;

    float _maxSlabX = (max.x - startP.x) * oneOverDir.x;
    float _maxSlabY = (max.y - startP.y) * oneOverDir.y;
    float _maxSlabZ = (max.z - startP.z) * oneOverDir.z;

    // Min/Max Slabs
    float minSlabX = Mathf.Min(_minSlabX, _maxSlabX);
    float minSlabY = Mathf.Min(_minSlabY, _maxSlabY);
    float minSlabZ = Mathf.Min(_minSlabZ, _maxSlabZ);

    float maxSlabX = Mathf.Max(_minSlabX, _maxSlabX);
    float maxSlabY = Mathf.Max(_minSlabY, _maxSlabY);
    float maxSlabZ = Mathf.Max(_minSlabZ, _maxSlabZ);

    float minSlab = Mathf.Max(minSlabX, minSlabY, minSlabZ);
    float maxSlab = Mathf.Min(maxSlabX, maxSlabY, maxSlabZ);

    // Check hit
    bool bHit = maxSlab >= 0.0f && maxSlab >= minSlab && minSlab <= 1.0f;
    if(bHit)
    {
        Gizmos.color = Color.red;
        int hitSurface = 0;
        if (minSlab >= 0 && minSlab <= 1)
        {
            // Draw first hit point
            ++hitSurface;
            Gizmos.DrawSphere(startP + new Vector3(minSlab * dir.x, minSlab * dir.y, minSlab * dir.z), 0.4f);
        }
        if (maxSlab >= 0 && maxSlab <= 1)
        {
            // Draw second hit point
            ++hitSurface;
            Gizmos.DrawSphere(startP + new Vector3(maxSlab * dir.x, maxSlab * dir.y, maxSlab * dir.z), 0.4f);
        }
        if (hitSurface == 0)
        {
            // line segment inside bounds
            Debug.LogError("inside");
        }
        else
        {
            // line segment hit surface
            Debug.LogError("hit");
        }
    }
    else
    {
        // line segment hit nothing
        Debug.LogError("nohit");
    }
}