How can you tell if a painting is an original?

Answer 1

The question is vague but here goes. A trained eye will always be able to easily spot original work from a copy.

If you're speaking about content however, you're pretty much out of luck unless you have documentation of some sort.

If you're speaking about art you own which was made by another artist, there are professionals with tons of knowledge on the subject which you'd be hard press to research on your own.

Answer 2

This site may be able to help you, excerpt from site: About Alan Klevit

Expertise

I can provide sound advice on buying, collecting & understanding fine art, especially original works by 20th century masters and leading contemporary artists. I can also explain techniques used to create original works and offer tips on how to tell whether or not a piece is an original work of art. * http://en.allexperts.com/q/Fine-Art-3122/tell-painting-bought-real.htm