E or not-E?

Electronic cigarettes have been banned on many trains and also in Wetherspoon's pubs it seems.  Well, it was only a matter of time, wasn't it?  Firstly, there are those who object to the idea of people getting pleasure from the ingestion of nicotine, however obtained, and maintain that it must pose a health risk to others, but I think the main problem is that it is difficult to tell - particularly from a distance - whether someone is smoking an e-cigarette or a real one - so do you challenge them and risk making a fool of yourself, or let it go and become complicit in a breach of the law? I'm not sure what the answer is, but I think there needs to be some way of being able to positively identify an e-cigarette and then most of the problem goes away.