Skirt darts tend to be triangular-wide at the base and then taper directly to points.
Shirt darts-at the bustline or from hemline to waist, the darts are not straight triangles always. The darts gradually taper (almost rectangular) then become pointed at the end. Sometimes pattern markings may indicate two sets of symbols before the pattern lines for the dart becomes pointed. You have to look at the pattern and the instructions to be sure.
From your question, I am assuming that you are modifying an existing garment for better fit or you intend to make your own garment without a pattern. My reason for saying so is that I adjusted a purchased shirt to fit on a family member (too much excess fabric in lower back) that needed darts. I had her wear the garment and I used chalk to mark my dart marks so I knew where to taper -not straight triangles at all due to body contour. Result was a shaped fit that helped to keep the shirttail tucked into a waistband.
