Scientists will usually investigate a subject from many perspectives through multiple experiments and experiences before they will draw a conclusion. Reiki deserves the same respect. Before Reiki is experienced, all one can truthfully say is "I do not know." You may have friends who recommend it and say they love it, but you have no experience yet. You may be skeptical about it, but you have no experience yet. And without experience, you just do not know.
On the other hand, maybe you have tried it, but you did not really notice anything from it. It is possible that nothing happened, but it is also possible that you looked left when something happened to the right. It is also possible you were biased against it working and did not allow for a truly objective experience.
Whether you are going to Reiki for the first or the hundredth time, in order for it to be truly objective, you must have no agenda, no preconceived notions, no biases, no judgments, and no expectations of any kind. So throw all of those out the window, and then you are ready to experience Reiki.
While you are receiving your Reiki session, become a silent witness. Feel the hands touching your body. What do you notice? Warmth? Tingling? Relaxation? What emotions or thoughts arise in your awareness? What images arise? Colors? Sounds? What sensations are in your body? Watch. Feel. Observe.
People's awarenesses of the Reiki energy are different. A few people experience nothing that seems unusual. Others will feel at least heat or perhaps an electric tingle. People who are more sensitive may see colors or visions or have more intense physical experiences. Other experience can happen as well. To those less sensitive to Energy it can be easy to overlook one very common experience which is a sense of peace and relaxation. This comes about because the vibrations of the Reiki draw the recipient into a higher state of consciousness: closer to meditative states.
After the session is over, one still needs to be willing to watch and observe. After all, people use Reiki to improve health and quality of life. So, we must observe how we feel after the Reiki Session as well. Do you feel calmer? Do you sleep better? Are you more patient with co-workers or family and friends? Is your pain less? We must be willing to lift every stone of our experience and look to see if there may be a change or a shift. Sometimes Reiki brings about dramatic changes and healing. Those are easy to notice. More often, it brings about subtle changes, and we must be present enough to see the effects.