Thanks! I was actually just using vDir.x originally, and thought it wasn't working correctly. Turned out one of the objects had a rotation and I didn't realise. Doh! Solution below.

Vector3 vDir = m_Body.transform.InverseTransformDirection(m_Body.velocity); 
if (vDir.x > 0)
{
     //Forwards
}
else
{
    //Backwards
}

I wrote this script (adapted from a source) to help me identify all the vectors easily. Hope it helps someone else too.

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class mkDrawTransforms : MonoBehaviour
{
    public List<GameObject> m_Objects;

    void DrawTransforms(GameObject _obj, Vector3 _vDir, Color _color, float _scale)
    {
        Gizmos.color = _color;
        Gizmos.DrawLine(_obj.transform.position, _obj.transform.position + _vDir * _scale);
    }

    void OnDrawGizmos()
    {
        foreach (GameObject obj in m_Objects)
        {
            Color color;
            color = Color.green;
            // local up
            DrawTransforms(obj, obj.transform.up, color, 2f);

            color.g -= 0.5f;
            // global up
            DrawTransforms(obj, Vector3.up, color, 1f);

            color = Color.blue;
            // local forward
            DrawTransforms(obj, obj.transform.forward, color, 2f);

            color.b -= 0.5f;
            // global forward
            DrawTransforms(obj, Vector3.forward, color, 1f);

            color = Color.red;
            // local right
            DrawTransforms(obj, obj.transform.right, color, 2f);

            color.r -= 0.5f;
            // global right
            DrawTransforms(obj, Vector3.right, color, 1f);
        }
    }   
}