There are many ways to track one’s progress towards fitness goals. Body composition scales, calipers, resting heart rate as monitored by an oximeter or pocket ECG monitor, and of course the mirror, all play their part.
Another interesting side effect of becoming fitter – particularly when combined with getting leaner – is increased vascularity. Most serious gym-goers will observe more prominent veins during their workouts, particularly during high rep sessions, as vasodilation kicks in to return the increased volume of blood back to the heart.
There is some evidence to suggest that blood flow restriction training (or KAATSU training) can increase vascularity, due to the pooling of blood in the veins as venous return is restricted, but this claim is somewhat controversial. The only guaranteed way to increase the visibility of veins under the skin is by reducing one’s body fat percentage through diet and exercise.