How do you tell the difference between correct ideas and incorrect ideas?

Test them in the external world through observation and experiment. Thus, Einstein’s “gedanken ‘experiment’” (i.e. “thought ‘experiment’”) is no experiment at all. The “ex” in “experiment” means precisely that—an adventure into the external world. We are all born as solipsists, thinking that we control the universe. As we mature, we necessarily find that this is not true. Some even go to the other extreme, becoming fatalists who believe the universe controls them. The truth is that the correct philosophy is univironmental determinism, the proposition that what happens to a portion of the universe is determined by the matter in motion within and without. At base, we need to determine whether or not ideas are correct so that we can negotiate the macrocosm in relative safety. I may have an idea that I can fly. I can test that idea by jumping off El Capitan and flapping my arms. Through observation of the experiments of others, however, I have learned that the idea is incorrect.

Other claims of truth or falsehood are logical deductions from acknowledged assumptions or hidden presuppositions. Nonetheless, until they are tested, such claims are no more likely to be true than the assumptions on which they are based. As part of the philosophical struggle, indeterminists often deny that one can distinguish between correct and incorrect ideas. Therefore, they claim that the Ideality of religion is not to be tested against the Reality of the external world. Thirty-five years ago such objections squelched an attempt to perform a scientific test on prayer (Brush, 1974). Recently, however, the test was performed and published in a peer-reviewed journal, albeit with an obfuscatory title (Benson and others, 2006) that was interpreted by an enterprising journalist (Stein, 2006). Nowadays, only the most backward folks rely exclusively on prayer in lieu of medical intervention. When children are involved that approach even appears to be illegal (Baenen, 2009).

References:

Baenen, J., 2009, Father in dispute over son getting chemotherapy pleads with wife to bring him home, Associated Press, May 21.

Benson, H., Dusek, J.A., Sherwood, J.B., Lam, P., Bethea, C.F., Carpenter, W., Levitsky, S., Hill, P.C., Jr., D.W.C., Jain, M.K., Drumel, D., Kopecky, S.L., Mueller, P.S., Marekk, D., Rollins, S., and Hibberd, P.L., 2006, Study of the therapeutic effects of intercessory prayer (STEP) in cardiac bypass patients: A multicenter randomized trial of uncertainty and certainty of receiving intercessory prayer: American Heart Journal, v. 151, no. 4, p. 934-942.

Brush, S.G., 1974, The prayer test: American Scientist, v. 62, p. 561-563.

Stein, R., 2006, Prayer doesn't aid recovery, study finds: Effect on healing of strangers at distance after heart-bypass surgery examined, Washington Post: Washington, DC.