With all the interbreeding that has been done it's getting harder and harder to to say that an indica or sativa is often a certain way. Generally speaking, indicas are usually darker green and leafier and so need more N than the average pure sativa. Here's an example from google images of roughly what I think of when I think of a flowering Indica.
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Now, the leaves could be a little fatter and shorter for a good indica example but you see what I mean about the leafyness. Here's an example of a sativa bud, again just grabbed from google images.
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It's much hairier than the previous plant with far fewer leaves that are a lighter shade of green. Combined with the frequently longer stems that sativas can have, all of the extra stigmas can require alot of K.