What to look for in foliage and leaf stems
One stem should not be one green
Real foliage changes colour along its own length. New growth at the tip is paler and yellower, older leaves lower down are darker and sometimes marked. A stem printed in a single flat green is the most common fault in this category and the easiest to spot once you know to look, because nothing in a garden is one colour.
Turn a leaf over
The underside is where corners get cut. A real leaf is paler and matte beneath, with a raised midrib you can feel and veins branching off it. Cheap stems are the same glossy green on both faces, which means every leaf that turns towards you catches the light identically and the whole stem flashes. If the listing has no photograph of the back of a leaf, that is worth noting.
Decide whether you want filler or structure
Fine, soft foliage goes between flowers and fills the gaps. Broad, stiff leaves are shape in their own right and work alone in a narrow vase. They sit in the same category because both are green, but buying the wrong one for the job is the usual reason a stem ends up in a drawer.