Start with the color - Fresh raspberries are vibrant red in color due to the high amounts of a phytochemicals called anthocyanins. A high quality raspberry powder should be vibrant red in color signaling that the anthocyanin molecule has not been broken down through the drying process. A powder that is pale in color has either been diluted with a filler (like the competitors product in the photo above) or it has been dried in a way that has broken down the anthocyanin molecule, in which case many of the other nutrients will have also been degraded. On the other end, if the powder is dark reddish/brownish, the powder has most likely been dried at a high temperature and was possibly burned during the drying process.
Visual seed fragments - Raspberries have a lot of seeds in them. If it is a whole raspberry powder you should be able to look closely at it and see small seed fragments in it (unless it is a raspberry juice powder).
Taste and smell - The taste of the powder should be tart with a subtle sweetness, signaling that it was harvested and dried at it's peak ripeness. If it is not 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 raspberries.
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 soil, or was it grown in highly polluted areas (such as China where heavy metals such as lead can be a major concern).