When injecting B12, it is important to know if you have hit a vein to ensure the proper administration of the vitamin. One way to determine if you have hit a vein is to observe the flow of blood. If blood starts to flow back into the syringe after inserting the needle, you have likely hit a vein. Additionally, if you experience resistance or pain while injecting, it could be an indication that the needle has entered a vein. It is important to be cautious and seek medical advice if you suspect that you have hit a vein while injecting B12 to prevent any potential complications.