Most people at one time or another will hit a vein. You need quite a bit of air injected to cause death, a small air bubble generally is not enough. If you inject into a vein depending on the amount injecting and speed you inject, you should be able to tell during the injection. You will feel a tightness in your chest when you take a deep breath, followed usually by uncontrollable coughing. The coughing and pressure will subside after several minutes.
Always aspirate, inject slowly and beaware of any unusual signs during the injection. Just because you aspirate and do not get blood does not mean you are not going to hit a vein. Veins are very small and the slightest movement can cause to to hit one. Hold the needle still and be cautious not to increase the depth during the administration. If possible have someone else do your injections they will have a steadier hand then trying to reach around those lats after a few cycles.