I'm sure if you'd asked 100 people, you'd get 200 different answers. So let me offer my two cents. ^_^
I go by feeling. Usually I can sense three things with lower dimensional beings or consciousness: (1) ego, (2) denseness/weight, and (3) disempowerment.
For me, ego is like the filter lens for a camera. It is a disillusionment.
As for the density, weight, or texture of the consciousness, the lower the rubbish is, the more it feels like rubbish. I think perhaps the best way to learn discernment is to ask for an interaction with higher energies, so that you can recognize them and feel the difference. But the kind of energies you attract is a reflection of who you are, so if you want to attract something really outside of your range, you kind of have to really go outside of your normal activities and thought patterns, to attract them.
Power is something that takes me a very long time to learn and to recognize. Certain energies or beings do not have a sense of disempowerment. I don't know whether they are always from higher dimensions or not. I just feel that they are very neutral (but loving) in the sense that they don't have control issues.
I remember once I read an encyclopedia on the different kinds of beings (gods, deities, etc.) out there, and some expect or require you to repay them with an offering if you recruit their service. I think those that require something in require might be of a different background, not sure if they are higher or lower, but for sure they are of a different lineage, and I think you would be able to feel that their consciousness is very different if you happen to come across them. I don't have much interaction with those beings, but I did sense a distinct difference when I speak to them.
I think what is most important to highlight is that your experience with energies is a reflection of your level of consciousness. If your level is high, it is rarer to feel something that is too low. Conversely, if your level is low, it is harder to feel something that is higher.