Girl try not to worry about it too much, most people don't notice things like that. Your arms are veiny because of several reasons, mostly because you have a low body fat percentage. Heat can also bring your veins out as well, and so can toned muscles... as being tone typically goes hand and hand with low body fat percentage.
I have the same "problem" my arms are veiny... I even have that big vein that runs down my biceps that bodybuilders so desperately want... but I don't =) Anyway, I'm really tone, i'm very veiny, and I went back to living as a boy/androgynous and wear tank tops everyday... and no stranger ever refers to me as a he. In fact, I've had several people come up and ask how I am in such good shape and tone for being a woman... but my original plan in wearing tank tops was to show off these veins and muscles and my flat chest so I would pass as a boy. So, the point i'm making is... most people don't notice it, and if they do they don't typically think "man!" they usually think tone woman. It's just like a that picture of Angelina that somebody posted, if you saw her walking down the street you would never think man, despite her veiny arms.