Ab:
Gb:
Here are four ways:
1) If the colour between the headlights is the opposite colour of the headlights, it's Gb.
2) If you see all four colours appear in order, it's Gb.
3) If the 2 far right colours are opposite colours, it Ab.
4) If the 2 inner colours on the right face are opposite colours, it's Gb.
Since these two cases only differ by the colours of the edges, you can find a way to distinguish them by the pattern of where opposite colours are/aren't. You can also apply this for Aa and Gd (mirror the directions). I find the first method the easiest since the face with the headlights stands out more than the other face.