I think I have an original painting by Henry Holiday, how can I
confirm if it is authentic and its value?

Well about the only way you can authenticate and value an artwork is
by consulting a professional dealer or auctioneer. Obviously, this
will be easier if you are located in a big city in the UK or US. You
don't say anything about your painting; but relevant points will be
whether it is signed; whether you know the history of the piece; and
whether it is in Holiday's general style. And on this last point,
Holiday is not usually regarded as an impressionist but more as a
pre-Raphaelite.