Scanning the aura or energy field before a Reiki session was actually a very big part of the original system as created by Usui. Neither of my teachers taught us this. It appears to have become lost in the Western way of doing things. Intuitively letting your hands guide you to work on areas of need was much more the norm in Japan, not just going through a system of rote hand positions, although doing so will cover whatever areas are in the most need and will bring about a very real sense of balance to the client, but will take longer and waste a lot of time. Time better spent on concentrating on the areas of most need.
To start with, you'll need to learn a few simple techniques to sensitize and open your palm chakras. One method is using a T'ai Chi and Qigong technique called "pulling taffy". Bring your hands up in front of you, palms facing each other about 3-4" apart and begin bringing the palms closer (but never touching) and then away (about 4-5") going out too far does something called "drops the ball" in T'ai chi/Qigong. Do this for a couple of minutes until you clearly feel energy between your hands, similar to what it feels like when you have two magnets and you push them together or pull them apart.
Another method is to take your right thumb and push it into your left palm and turn it clockwise with gentle force several times. This opens up the minor palm chakra. Repeat on right hand using the left thumb. The last method is to put a drop of lavender oil into the palm of each hand - again this opens the palm chakras. After doing this, you can starting scanning or do the "pulling taffy" technique for a minute or two until you feel the sensations as I described above. Now you are ready to scan.
I personally like to use this method - I am right handed, I bring my hand into a this position - palm facing out to the left vertically over the body at the feet. My left hand is palm up to my left side and out just a little ways from my body. Then you begin running your right hand through the person's energy field starting about a foot away from the soles of their feet. Only scan up the top most part of the thigh first on each leg. You are looking for feelings of heat, tingling, or resistance.