What to look for in daisies and cosmos
The stem does half the work
These are small-headed flowers, so what you actually see is mostly stem. It should be fine, slightly bent and a little uneven in colour — a thick uniform stem under a small flower is what makes a cheap daisy look plastic even when the head is fine. Cosmos in particular grows on almost wiry stems.
Cosmos is not a daisy
A cosmos has eight or so broad petals, notched at the tip like they have been cut with scissors, above feathery thread-like foliage. A daisy has many narrow petals and simple leaves. They are grouped together because they do the same job in an arrangement, not because they look alike, and a good version of each is judged differently.
These are the hardest ones to fake
Fine, papery petals glow when light passes through them, and no fabric quite reproduces that — which is why backlighting is where daisies and cosmos give themselves away. Place them against a wall rather than a window, and let a stronger flower lead while these do the filling.