Holistic therapists have various ways of measuring a patient's energy flow and identifying blockages or imbalances:
Acupuncturists gauge the energy of the meridians through the pulses.
Ayurvedic Practitioners also gauge the energy of the meridians through pulses.
Kinesiologists test the muscles in relation to the energy system.
Reiki Practicioners may sense or even see problems latent in the energy field. Weaknesses or imbalances in the chakras can be sensed by the hand of a skilled practitioner.
Shiatsu diagnosis is made by feeling for the flow of Ki or Qi energy at the site of acupoints, along the meridian lines, in the area of the abdomen, and at the pulses. 
Healing
In this context it is defined as 'whole again'.  Reiki restores the original blueprint of one's health each time it's applied.  It successively restoring the image into the molecular memory within the cells of  the body to imprint and manifest as whole and healthy again.
                          It's important to understand that Holistic bodywork is like peeling away layers of an artichoke, eventually you will get to the heart.  Working through the layers allows you time to digest and integrate the changes that occur.  Even if I were powerful enough to cure you in one session you may not be capable of integrating that change therefore you might energetically "kick out the healing" or reject the healing.  The changes must occur at the pace that is right for you; a pace that you can integrate the changes into who you believe you are.  This is the way true healing and balance occurs.
                Something important for all therapists to realize  is that lives will be taken by disease.  There is no way of knowing what the life lesson is for that soul.  Maybe the individual's higher self needs to learn to accept help and assistance from others.  Maybe she needs to embrace impermanence or find acceptance of the reality of life and death.  We as outsiders can create peacefulness around the individual by trusting that the circumstances this person faces is what's necessary for them to learn exactly what they need to learn.  Our acceptance of this will allow us to relax and be calm in their presence.