Image stealing tends to go two ways,
1. GIANT words across the image, for some reason
2. Photoshoped out background. Usually it's pretty easy to tell, the image will usually be in different focus than the background or the edges will be rough.
Sometimes they won't do anything, and the only way you can tell is if you can identify it from a brand.
Baby The Stars Shine Bright / Alice And The Pirates
Everything is photographed front-on. Red velvet background, except items in colors that wouldn't contrast well (like red), which are on pink.
Mannequins have white cloth skin with no head or arms. Hats are displayed on a stand with no hair, accessories have the same background as clothing.
Shot front-on against a white wall, white rather nondescript mannequin with pink neck top. Hats and hair accessories are on a head display with a brown wig.
Accessories are on pink/white gingham or polka-dot fabric (sometimes with creases). Same white wall in background.
Mannequin has branding on the neck and base, no arms. Neck topped with silver metal. Plain creme wall in the background. Hats and bonnets are photographed on a stand with a brown wig and no facial features.
Accessories are shot on red fabric (similar to Baby) or what appears to be a fake wood table.
Clothing usually photographed angled slightly to the right. Mannequin has branding on the neck and base, a wooden neck. Background is usually floral wallpaper in brown or blue, or a white textured wall. Wooden arms are used in set shots.
Photographs featuring models usually (but not always) cut off below eye level. Hat stand has closed eyes and a brown wig that is usually curled, sometimes braided.
MM has a weird habit of using photoshop to change colors instead of taking a photo in each colorway, and sometimes they're not too great at it.
Victorian Maiden / Beth
Several different wallpapers as backdrops. Mannequin has white fabric covering and wooden neck piece, usually facing towards the left. Hat stand has several different wigs, including this one and a brown hime cut.
Some long sleeved tops are shot with arms, some not. One mannequin has leg stumps.
Juliette et Justine
JetJ uses a whole range of different backdrops, but it doesn't really matter since they always use a gold border which should be visible even when cropped. The third one is the most common, but there are a few variations.
Pieces are shot front on, occasionally turned to the left. Various floral wallpapers as a backdrop. The mannequin's neck is usually cut off in dress shots.
Ribbons on hats and bows are curled. Although they did have other models they seem to use this one the most.
Moi meme Moitie (courtesy of kagaminouso )
Moitie's photos tend to look like they were taken with a cell phone camera. They're usually pretty grainy.
Clothing is usually photographed in front of a blue fabric background, and accessories are usually laying down on a blue surface. Sometimes the clothing is shopped in front of a plain blue background, but most of the mannequin should still be visible.
The mannequin is covered in a textured fabric, as you can see here:
It also usually has a wooden top, though this is sometimes cropped out. Another mannequin with a black metal top is sometimes used, and it appears to be covered in the same fabric as the other one.
Also, the metal-topped mannequin has leg stumps, where the wooden-topped one does not.
Things are almost always photographed front-on
Fan Plus Friend
F+F stick a HUEG watermark across everything that can't be cropped without cutting off half the picture, so it doesn't really matter.
Bodyline uses a few different mannequins, but they usually use the same box format to get several different views of the item. Models are usually posed with a red/gold couch and a concrete wall in the background (a white wall in older products).
(Although if someone is using Bodyline images they could just be selling Bodyline.)