Aaah, the much desired cephalic vein aka biceps vein. Yes, everyone should have a cephalic vein, so don't worry about that :P
1. Now first make sure you're indeed lean enough. You have a well defined sixpack? Then you're percentage of bodyfat should be low enough for the cephalic vein to show through.
2. Make sure your biceps is huge! My right biceps is slightly larger than my left, supprisingly my right cephalic vein is more prominent than my left. Conclusion: the biceps muscle may act like some sort of pump making the cephalic vein appear larger.
3. Genetics. Sadly, some things we can't change (yet).
"Knowledge is the fuel. Training is the spark"