Look for dark green color and tight florets. If the florets look like they are beginning to open, the more mature the broccoli is, and that's not such a great thing. The broccoli should be pretty much odorless and the stem should be firm and crisp. If the broccoli is yellow, has black spots or mold, is soft, or has a bad odor, it's probably not any good and should be tossed or composted.