quote............"Is there a way to verify if it is gilt?"...... yes, scratch it, and assuming it comes off, then it's, gilt - although wouldn't recommend doing that anywhere it can be seen.
Originally, ormolu was an alloy of copper, tine and zinc, producing a gold coloured metal throughout its thickness - but like everything else in life the cost cutters got to work and the product ended up more usually as gilt on brass or bronze.
I'd agree with m, and consider this to be opalescent and not opaline - it has less opacity than opaline. It may indeed be French, but equally might not be.
Some very attractive Venetian revival style glass was produced in the U.K. during the 1870 - 90 period by people like Salviati (who appears to have influenced Sowerby) and Harry Powell - most pieces of which have in common some variation on a theme of pale milky/bluish white colourway, and all of which seem to have been described at that time as opalescent.
Hope you don't mind, but I've added couple of pix of an opaline turquoise finger bowl which, when compared with your piece, show the higher level of translucency of your opalescent epergne, when compared with the more opaque opaline.
Pieces like this with their wavy thrown rims may well owe their design pedigree to the fashion of the Venetian revival period which I've mentioned.
Sorry, but the GMB tend not to provide values for glass - these things are too personal to be set in stone. You might try ebay or look at professional sources such as auction guides etc.
P.S. If I have any of the above wrong, please correct me as soon as possible - thanks.