OK...do you mean a real painting from a giclee on canvas or something? Or the original from a forgery? Or the original from a Master Copy? There are also "houses" that specialize in accurate, painted reproductions. I mean, if it's good enough only a highly trained expert could tell, and even then it is sometimes debated. Chemical analysis often comes into play in those cases.
If you just mean an actual painting vs. a print that is pretty easy - the print will lack any impasto brushwork. In the case of a painting with no impasto, like a Bougeurau, it would actually be easier - because you don't have an original Bougeureau.
Edit: Tell 'em I said that's how you tell! Actually really frowned on in museums though.
What would Caravaggio do?
And then God said, "Let us make man in our likeness and our image. Let us make him ridiculously hard to draw so that poor artists everywhere will have to spend 10,000+ hours failing repeatedly before they can begin to capture the form and likeness onto a two-dimensional surface." And there was man. And it was good. And artists everywhere lost their minds.
There was a good program on forgery on PBS. It involves gathering materials from the same time and working in the same methods as the original artist. The forgery is so accurate, not even chemical analysis will help.
In the end it boils down to brushwork and method analysis. To give you a crude example, if you know that a particular artist X preferred to work on gray ground and in layers and smoothed his brushwork along the form, and the painting you have is on white ground and alla prima and mostly across the form with a glaze on top, then it's probably not by artist X. If a forger is skilled enough, it gets progressively harder. (Especially if they use the same material types - some of them will take insignificant paintings from 19th century and paint forgeries of a more acclaimed artist on the canvas, so the canvas is genuinely old.)
LOL - glad you think so kevan (may I call you kevan? Those extras in front confuse me).
Yeah - it would get pretty tough with a highly competent forgery - I didn't realize they even had tricks like TA and arenhaus mentioned - but it makes sense. I assume that course of study would be in an art conservation/curator program?
What would Caravaggio do?
Once a painting gets old enough, the main way of determining authenticity comes from documentation - we read through records of what famous painters made, and who they sold it to, and try to follow a paper trail. Of course, a good forger can make this up too. One of the tricks is to read through the literature, like Vasari's Lives of the Artists, determine which artworks are still missing, and then try to make one yourself, in the same style. What you have on your side is that so many curators and enthusiasts want to believe you.
Actually I was listening to a podcast where they mentioned that one of the most lucrative parts of art theft is actually reselling originals as extremely accurate reproductions... essentially, rather than waiting for the statute of limitations to expire or for a black market dealer to purchase it, they move the artwork as a copy of itself and sell it for much less money, but its a lot safer. Apparently this happened to Speilberg, where he thought he had an extremely accurate reproduction of an original Rockwell, only to discover it was the real deal...
Uhh, my point with all of this was just to say that, yeah, apparently forgeries can be that good.