Start with the color - Fresh spinach leaves are vibrant green in color due to the high amounts of chlorophyll. A high quality spinach powder should be vibrant green in color signaling that the chlorophyll molecule has not been broken down through the drying process. A powder that is 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 spinach leaves are light and fluffy. A high quality spinach 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 spinach has 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 spinach powder which is light and fluffy, compared to the same weight of a competitor's spinach powder which as you can see is much more dense).
Taste and smell - If the powder doesn't taste like spinach, 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 spinach.
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).