Start with the color - Fresh broccoli has a light green stalk with dark green "beads". If you take a close look at the powder, you should be able to pick out these different colors. If you cannot, the powder has either been diluted with a filler, or dried in a way that has degraded the chlorophyll molecule in the product. If that is the case, it has also degraded many of the other nutrients in the broccoli.
Density - Fresh broccoli is about 90% water. When you remove that water and maintain the cellular structure, the powder will be light and fluffy. If the powder is dense and heavy either a dense filler has been added to the powder, or the broccoli has been dried in a way that has broken down the cellular structure in which case many of the nutrients will have also been destroyed (the picture above shows our broccoli powder which is light and fluffy, compared to the same weight of a competitor's broccoli powder which as you can see is much more dense, not to mention very pale in color).
Taste and smell - While broccoli is not the most pleasant taste to most people, if your powder doesn't taste like broccoli, it has either had a filler added to it to dilute the taste, or the flavor molecules have been broken down through the drying process, in which case so have many of the other nutrients. A high quality powder should look, smell, and taste like broccoli.
Other things to consider - Lastly, when evaluating the quality of a powder it is important to understand how and where the product was grown. Was it grown using Organic farming practices and is it certified USDA Organic? Was it grown in good quality USA soil, or was it grown in highly polluted areas (such as China where heavy metals such as lead can be a major concern).