whats the best way/clues that a painting is original and not a painted reproduction?

Answer 1

It depends on the painting. If it is something well known than there are records of the original. Usually the frame is taken off and a photo is taken of the picture that nobody seen, since nobody has seen it nobody can copy it.
If the painting is not very known styles are compared, brushwork and so on.
It is very hard to tell the reproduction from the original sometimes because the people that copy the painting might be using old canvas or wood that the painting was painted on, than make paints that are the same as the original, even go as far as make the same brushes. They use the same techniques to paint it too.
I hope it helps

Answer 2

The price tag.
The sides. If they are 'clean' it is almost certain a copy.
The age of the frame and the canvas.
The smell.

Oh, best one: Ask somebody who knows and who you trust. Like an art action house.