Proto wrote:

The ligit cigar sellers will offer to smoke one with you before you buy.
The ONLY way to tell for sure. There used to be a site that showed the
labels next to the couterfit ones so you could tell but then you would have
to really study them or carry a photo book with you and then argue with the
guy if he insisted the photos were wrong and he was right. I decided to give
up smoking

If anyone is interested in the science of fake cigars, this site is
worth a look:

http://www.cubamall.com/spotfakes.html