It's largely a matter of "leanness"
If your body fat % is in the ~10-12 range, you can usually get a few leg veins to pop by flexing.
If you're in the 8-10 range, you can even pop ab veins.
But there's a good reason to go for the arms. Unless you're particularly obese ( +25% ), you can usually get an arm vein to pop by repeated flexing & then tying off.