Dam Monkey trolls are rather uncommon (I dislike to use the word "rare" too much, since it is used without discretion often on eBay. I try to only apply it to things that are truly rare - as in there are only a few in existence), and extremely exciting to find for the Dam collector.
Monkeys come in two sizes: adults, who are about 7.5 inches tall (measured to the top of their foreheads), and babies, who are about 3 inches tall. The adult male monkey is more common than the other two, and the adult female is the most uncommon.
Since girl monkeys are so hard to come by, sometimes people like to redo boys into girls. The way you can tell if it is a real girl monkey is whether she has slits on the sides of her face. Boys (adults and babies) have slits in their vinyl on the sides of their faces for the mohair of their sideburns. Girls do not have slits.
In my opinion, the more wrinkled the troll face, the better! Monkeys have the most wonderfully detailed faces!

Monkey trolls are also known for their detailed hands. They are the only other troll besides the vintage tailed troll to have fingernails. They also have wrinkles for the knuckles, which is unique. The adult monkeys are the only ones to have these characteristics though - the baby monkeys do not have hand details.
Original attire
The baby boy's original costume is felt briefs, and a sailor hat (which my little guy doesn't have - its the first thing that goes missing). They have short, fluffy mohair, and little sideburns:

Girl baby monkeys are definitely rare. I have only heard of one person who actually has one; Stuart, one of our resident Monkey troll connoisseurs. He was very sweet and let me use a few of his photos :-). The girl in red on the left is the real girl, the one on the right he thinks started out life as a boy.
Girl baby monkey's original costume is a felt dress, and they have long mohair:

(photo used by permission)
Adult girl monkeys original costume is a red and white striped jumper, with a collar that folds down, and a white skirt. They have long mohair:

The adult boy's original costume is a blue and white striped jumper, with a collar that folds down, and a sailor's hat with red pom pom. The hat is commonly missing. They have short, fluffy mohair, and sideburns:

Raul's Monkey trolls
My boy monkey pictured above is holding up a Rauls Swinging Monkey Troll. Rauls' animal trolls are either covered in rabbit fur, or have it for hair. They also usually have a string from which they can be hung, and are made of more squishy vinyl. Below is Rauls' sitting monkey troll. The swinging monkey is about 3 inches tall, and the sitting monkey is about 4 inches.
Monkey's Dam Stamp/Engraving:

Those addictive Monkeys!
Dam monkeys have such wonderful character, it is really easy to fall in love with them! Some collectors have become connoisseurs of the monkey troll, such as Stuart or Vicky in England. Below is a photo of some of Stuart's fabulous monkey collection. Look at all the varying skin tones and mohair! The monkey in the middle, on the second row wearing burlap overalls is the extremely rare young monkey troll! There are only a very small handful of these trolls out there. How neat!! It seems like fate that Stuart should have him :-). Also notice the little swinging Rauls monkey at the bottom.

(used with permission)
To see more of Stuart's awesome Monkey Troll collection, go to his Flickr page: http://www.flickr.com/photos/trollcatch22/