Start with the color - Fresh collard green leaves are dark green in color due to the high amounts of chlorophyll and the stems/veins are lighter green. A high quality collard greens powder should be vibrant green in color signaling that the chlorophyll molecule has not been broken down through the drying process. You should also be able to pick out flecks of pale green in the powder which is the stem/veins. A powder that is completely pale in color has either been diluted with a filler or has been dried in a way that has broken down the chlorophyll molecule, in which case many of the other nutrients will have also been degraded. On the other end, if the powder is dark greenish/brownish, the powder has most likely been dried at a high temperature and was possibly burned during the drying process.
Density - Fresh collard green leaves are light and fluffy. A high quality powder should also be light and fluffy. If the powder is dense and heavy either a dense filler has been added to the powder, or the collard greens have been dried in a way that has broken down the cellular structure of the leaf in which case many of the nutrients will have also been destroyed (the picture above shows our collard greens powder on the right which is light and fluffy, compared to the same weight of a competitor's collard greens powder on the left which is clearly much more dense).
Taste and smell - While collard greens are not the most pleasant taste to most people, if your powder doesn't taste like collard greens, 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 collard greens.
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).