How can you tell if a painting is an original?

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.