Oh, there are numerous ways whereby we can test the possible effects of thought on reality. The basic idea simply goes as follows:
1. An experiment is done. The results are measured. (This is the control group).
2. The same experiment is done again, under essentially the same conditions, except that one additional variable relating to thought is introduced. Again, the results are measured.
In this way, we investigate the possible effects of thought on reality. Any difference in the measured results would / could support the hypothesis that the additional variable had caused the difference. That's basic experimental methodology in science.
Simple illustration. We get a large number of patients, suffering from similar same medical condition / disease. We adjust for variables such as age, severity of symptoms etc, and we divide the patients into two groups.
Both groups are given the same kind of medical treatment. However, one group is asked to do some kind of mental exercise, eg meditate, be hypnotised, visualise good health etc etc.
We then monitor both groups and see whether there is any difference in how quickly the two groups recover from their medical condition / disease.
It's not as if this kind of stuff hasn't been done before. For example, in a 2003 study, researchers from Harvard Medical School found that hypnosis helps broken ankles to heal much faster (on average, six weeks faster than the control group).
In a follow-up study, also by Harvard Medical School, researchers found that hypnosis helps patients with surgical wounds (following surgery for breast cancer) to recover faster.
Harvard Gazette: Hypnosis helps healing
So I trust this example makes it clear that there are certainly many ways whereby the effects of thought on reality can be tested.