How to tell Gourd from Wood on estate Calabash

I wouldn't buy one over the internet except, possibly from a reputable dealer who will know what they are selling and stand behind it. Wood looks nothing like a gourd, especially inside. I'd want to personally inspect it.
If you are limited to internet buying, eBay and the like, I'd closely review the feedback regarding the seller's reputation. Weight, lack of grain, interior roughness or "fuzz" are the indicators I'd look at when inspecting a calabash. Gourd is lighter than the woods, usually mahogny, used in "gourd" shaped pipes. Wood has a distinctive grain, gourd is a fruit related to the pumpkin. The interior of the skin is "fuzzy" or rough, similar to the inside of a pumpkin rind. Pumpkin is a gourd.
Also, you want to know if the bowl is meerschaum or porcelain. Meer is traditional, porcelain is usually cheaper and, in my opinion, does not provide the smoking experience of meer.
That's pretty much the limit of my knowledge on the subject. Hope it helps.

Several guys here are quite knowledgeable & have nice gourds in their collections. I'm sure they will chime in. In the meantime you might find these threads informative with plenty of porn!
http://pipesmagazine.com/forums/topic/the-gourd-calabash-lovers-thread
http://pipesmagazine.com/forums/topic/question-about-gourd-calabash

That pretty much covers it Warren; light, no grain but maybe some darker mottling, rough inside. It's not too difficult to tell apart as long as you have good pictures.
This is gourd:


Mahogany:

Both look gourd to me, but the second looks like it has a porcelain bowl.

The thing about replacement bowls is that no two gourds are exactly the same size, at the factories inserts were often made to approximate sizes then turned to fit each individual gasket. I recall finding at least one online vendor who will do this for you if you have exact measurements, otherwise you can try replacing the gasket with a thinner or thicker cork piece to get the right fit.
That second one is a porcelain bowl, I've got a similar Pioneer that shined up nice but haven't tried smoking it yet.