Im at work and i sliced the tip of my thumb with a knife by accident, how do i know if it needs stitches without going to the er first?

These excerpts from Healthline website can answer your question: "Your wound likely requires stitches if: it’s deeper or longer than half an inch; it’s deep enough that fatty tissue, muscle, or bone is exposed; it’s wide or gaping" and "A laceration that is bleeding profusely and doesn’t stop after 10 minutes of direct pressure likely requires stitches".