Pull back very slightly on the plunger of the syringe.
You will know the needle is in the vein when blood flows back into the tubing.
If there is no blood return, feel for the vein without removing the needle. You can try to move the needle toward the vein if you do not try more than once or twice. Poking around can injure the tissue around the vein.
If there is still no blood return, remove the needle and press down on the site for five minutes. Try another site using a new needle.