I have a blue vein running down my left bicep but not on my right one. I have been told before that a vein on your bicep means you are losing fat, but should they be on both biceps or just one? Is this normal?
This is caused by genetics along with childhood development. Possibly you used your one arm more or differently than your other arm causing the vein to rise to the surface; or possibly it is merely genetics.
If you work out intensely over years then you may find that the other arm will encounter raised vein(s) as well, although possibly not in the same location.
It is definitely normal. The veins on my biceps are not symmetrical either, and this is the same for many if not most people. Nothing to worry about! If you want symmetry, however, there is not much you can do besides working out intensely over years.
Also, veins on the bicep don't necessarily mean low bodyfat, for example I've been 8% bodyfat and also 24% bodyfat and in both cases, I did not appear fat and veins were on my arms. It has a lot to do with genetics.