how can you tell a real grete jalk coffee table from a replica?  

My experience has been to...
My experience has been to the contrary expo67. I know of only one or two shops out of the myriad of others in Toronto who's prices are consistently reasonable for good quality Danish / mid century furniture.
A person's definition of what is a reasonable price is another matter.

One or two reasonably priced...
One or two reasonably priced shops is all you need. I can guess (maybe) who you mean. The other shops buy from them as well. I guess it all depends on what you see as the retail price benchmark to compare to - which shop, which online source, or whatever. With a little knowledge, a list of shop sites, and a phone, it's not that hard to find 'better' prices. I do tend to avoid the big city nonetheless; prices are usually always better in the burbs and at the shows.

7 Slats Jalk
Here's my table, which I have seen others selling as Grete Jalk "European version." My table measures 51 inches long. Note the Danish Control label.