Whatever you're looking at, it's most likely a '80cc' kit.... There aren't really any sellers who have the 49cc kits anymore (because no one ever buys them).
There have been past reports of some ebay sellers listing 49cc kits & then shipping 80cc kits to get around local laws. (so be careful of this).
To answer your question; You can tell by looking at the head; 49cc engines have smaller heads (but you have to have a little experience to tell the difference).
Another thing to consider is that the picture listed might not be of the actual item.