Look for tight heads that are dark green. If you see any yellowing don’t buy it. Also check the ends of the stalks. They should be smooth and pale green. Reject any with holes in the core or with a woody appearance.
If your store sells broccoli crowns as well as full heads always compare prices. Weigh a full head to see what it’s actually costing per pound. For example, the other day heads of broccoli cost $2.58 each. I weighed the head I had selected on a nearby scale and found it only weighed a little over a pound, so the cost was more than $2.00 per pound. Broccoli Crowns cost $1.58 per pound that day and they have less waste. I put the full head back!