No "possibly" about it....undoubtedly by Fenton. The pattern has not been reproduced or faked (to the best of my knowledge).
You need to check for damage, as you said, Pip. That is the most important thing - plus the quality of the iridescence. You said the base isn't lustred. I am not totally sure of what you mean. Are you saying that when you turn it over and look at the underside, then that part has no iridescence? If so, then that's fine.
The base glass is just plain green glass. The iridescence is sprayed on to the hot glass, thus giving the "Carnival" effect. A good piece will be iridised all over the main part (top) of the item. If it is patchy and worn with bits missing, then don't get it.
So, no chips, dings, cracks or missing iridescence? Then it should be OK.
Green D&Ls are nice items, and oddly do seem to turn up more in the UK than in the USA (pro rata that is).
You can see some opalescent D&Ls here - they are very sought after. If you ever see one of those, get it!http://www.geocities.com/carni_glass_uk_2000/fenton_opal.html
If I have not answered all your questions fully, please ask me some more.