The only sure way to tell if you have E glass is to use an electronic tester. Your fitter will likely have one and should be happy to pop in for 5 mins to confirm it is E glass, they're about £60 I think. If they claim to not have such a device or are reluctant to come back that may be a clue. Did they issue you a Fensa certificate?
Building Control inspectors will also have such a device.
Allegedly to the trained eye there is a sheen that can be seen or something like that, from the Pilkington website: " Pilkington K Glass has high light transmission and appears virtually the same as clear float glass. However, in rare instances of strong oblique lighting, the coating may be seen as a transparent film. This is simply a transient visual effect which can be considered positive evidence of the coated surface being present. Further evidence of the coating's presence is through the very minor effect it has on white light transmission. This effect is so small as to be generally unnoticeable However, when a light coloured object or material is in close proximity to the glazing, dependent on local circumstances and conditions, a slight darkening can be noted."
Normally the glass has sticker on it as it arrives on site which are removed by the fitters. You obviously do not have these. Realistically it won't really make any difference but if you've paid for it you should have it.