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First, have a look at skin shadows and highlights. Do they look soft or hard? This is the first giveaway on whether a hard or soft light source was used (hard = small direct light, soft = large bounced or softened light).
Next, check the eyes. If they were not ´shopped away, you can see the main light reflect as little white spots.
Example: Top middle shot shows little pointy highlights on the skin plus a small spot of light close to the middle. This is obviously direct flash, somehow positioned close to and left of camera.
Note: top right picture says "could be a window", but in fact it can´t. If you look closely, you will see that the reflection grows bigger on its top although it should grow smaller, since the eye is shaped like a ball. This means, either the model was so inclined to that "window" that she was almost lying flat on her stomach or there was a softbox that was inclined toward her head. And that´s what it was. Nonetheless, softboxes DO reflect the same way a window with a drape does. So it remains true in a way.
Checking out the pupils, you can also see whether the shot was made in a studio or outdoors (sometimes you can tell this way only, because the image may be cropped or edited into another background). Wide open pupils will tell you that it was pretty dark and probably indoors. The lower left and top middle shots show a daylight pupil, while the others (especially top right) were obviously made indoors. The eye doesn´t react to flash, because it´s way too short, so unless continuous lighting was used, the pupil always shows the natural conditions around.
Many fashion shots are done with standard beauty light: one large softbox, beauty dish or umbrella high above camera and a reflector right out of frame below. Examine your favourite lingerie catalog for the following examples (badly forged by myself with my lousy mouse drawing skills):
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