Just look at the picture borders in live view, go outside and straight from the camera away for a few meters (e.g. 10m)...then measure a) the distance to the camera and b) the distance from that point in parallel to the border of the live view. Now do some easy mathematics: tan(x)= b/a. After calculating x, multiple it by 2 and you have the horizontal aperture of your lens in degrees (e.g. 70°).
Now it is easy to find the lens what is mounted (data is for a 1/3" Sensor):
2.8mm: ~90°
4.0mm: ~70°
6.0mm: ~50°
12.0mm: ~26°
Have fun!