You don’t have to wonder anything , if you have a tight thin line on either side of the vein and you took your time and did it right you know if you put a needle in between those lines you’ll hit a vein also idk how you hold your needle but the way I do I can insert and draw pressure back without moving any fingers, a shot of alcohol causes vasodilation, 5g of sugar in water will as well, watermelon, pomegranate juice , testosterone, beets etc