When I first started collecting gashapon, I had no idea what I was doing or what to look for. I ended up with a lot of poor quality bootlegs and even though I thought they were awesome, once I got my hands on some real ones I quickly saw the difference.
I put together these pics and a detailed description for you to point out some of the things I've noticed. If you're already familiar with gashapon, this might be nothing new to you. But if you are like me and you have no idea what you are doing, maybe it will help! :D
For both pictures, the real one is on the left and the bootleg is on the right.
Biggest thing to look for (if the pics are good enough): bootlegs do not have "stamps" on them.
Stamps are imprints of the copyright information and they all say: © T.P.TA.T B. CHINA and then there are four dots that form a diamond. Usually on gashapon with sailor fuku on, you can find this stamp on the back of their sailor collar and bodice. For the Eternal Sailor Moon shown above, you can see that it is imprinted on the inside of her far leg. For the Usagi with ice cream figure (and the Ami in pajamas figure) I think the stamp is only found on their stands. Princess Serenity has this mark on the back bow of her dress, and Luna and Makoto with her lunch have it on the bottoms of the figures. Rei in her shrine outfit and sitting Sailor Venus have it on the underside of their hair, so sometimes it's tricky to see if the figure is put together.
The other biggest thing in spotting a bootleg is, of course, quality. Many bootleg sellers use stock photos. When I'm buying gashapon, I tend to look for listings where people are posting photos that look 'just taken.' When I'm buying on LJ or through a message board, I'm a little more flexible on that rule since these people are actually in the community, but if you can get an actual photo of the actual one that you would be buying, that can be helpful. I'm not shy to ask for a photo of the stamp if I'm wary of a seller.
The bootlegs usually don't come in as many parts. Take my ESM for example: her skirt is three separate layers (and a pain to put together xD). The bootleg has all the skirt layers as one piece attached to the torso, and you can see a mold seam along the side of her body that tips you off they were all cast in one piece. For Usagi with ice cream, her belly plugs into a hole under her shirt and the skirt doesn't touch the belly. On the bootleg, it's all one piece and that space is filled in.
The bootlegs will have sloppy, more pronounced seams and lots of imperfections in the surface. If you look at bootleg ESM's near leg, you will see little holes in her boot. That is where popped air bubbles left a hole when the casting was made. Real gashapon do have subtle imperfections along the seams, but not usually big air bubbles like that on smooth surfaces. Also, notice the change in direction of the seams in places like ESM's sleeves. If you can find a photo of a real gashapon to compare the one you might buy to, you could see something like this.
Taking a look at Usagi with the ice cream, a few more things become apparent. For one, the color is terrible. Now this could be due to someone being bad at photography, so that's not a key factor but it could be useful when combined with other things. Also, check out the placement of her arm and legs. On the real gashapon, her arm is way back and her legs are open in a walking stance. In the bootleg, her arm hangs limply by her side and her legs are wonky.
You can also look at the details. How do the eyes look? Are they very crisp and clear, or kind of grayed out? Little details like the creases in the hair and earrings can get lost when a mold is made. If you look at the bangs on each you can see the real gashapon have nice, crisp edges while the bootlegs are kind of fudged. The details on her brooch are another strong give away.
I hope that has been helpful and informative! :D Happy shopping!