How to know if I need stitches for the cut on my finger?

STitches. In general may need stitches if wound depth > 1/4”, wound edges are gaping or jagged or if fat / muscle is visibly extending from wound. If there is any loss of movement - that will certainly require examination. See: http://www.mayoclinic.org/first-aid/first-aid-cuts/basics/art-20056711.