When chakras begin to open, or flower, it can be felt in the body. Different chakras feel different as they are opening/ activating. When it happens, you will know it, as it is impossible to miss.
if you feel at one with the attribute(s) the chakra is associated with, then it is open...
you might also just feel a pleasant sensation at that part of your body if you can visualize it really well, brightly or otherwise intricately it probably open it definitely helps to visualize them, and think of the associated feelings