In brief
Dark green tips and hard stems are landmarks when choosing. Separating the crown into petals facilitates preparation.
Useful to know
Look for broccoli with dark green tips and hard stems; avoid yellowed heads. These are the specific guidelines of the University of Iowa for choice. Store the product unwashed in a non-sealed bag and rinse before use. [1]
For preparation, separate the crown from the large stems and divide it into florets of a convenient size. [1] If you use the broccoli in paste, the size should allow the vegetable to be mixed and served with the other ingredients without having to cut into the plate.
One practical task is to review the resulting parts before cooking. You can split the too-big ones instead of leaving a few completely different sizes in one batch.
Don't assume that the biggest head is automatically the best fit. The amount only makes sense according to the number of servings and the plan of use. Buying more does not save a product if some of it remains unused.
Sources
For further detail and to check the information: