Most people have visible veins on their hands. That's where veins are... there's not a whole lot of room in the hands, and they're on the dorsum of the hand, just under the subcutaneous fat. If you're fairly thin, they're more easily visible.

If they've suddenly gotten more visible, you may have lost weight, have higher blood pressure, perhaps are keeping your hands "down" a long time (and gravity is pooling the blood there).

I wouldn't sweat it.