Whether or not to test for an allergy is a question that should be decided between the patient and doctor. However, it's good to know a few things in advance when seeking medical advice.
What is an allergic reaction? Basically, when the body, specifically the immune system, fights off an invading food item after initially (and mistakenly) assessing the food as dangerous. If the antibody immunoglobulin E (IgE) is created, a standard allergy test will be positive. It is believed that a person who tests positive has a genetic predisposition for allergies.
However, as many sufferers will attest to, there are other ways to suffer from your body fighting off a food! There is food intolerance, in which the body lacks an enzyme to digest the food. Uncomfortable? Yes. Life altering? Probably. Dangerous? Well...maybe if the person is at risk for dehydration. Then, there is an area in between the standard allergy and intolerance, often referred to as a sensitivity, delayed onset, or non-IgE allergy (there is a good explanation of this here). It has basically the same symptoms and issues as a food allergy or intolerance, and is sometimes medically diagnosable.
In the end, testing to determine whether your have an corn allergy, delayed onset corn allergy, corn intolerance, or whatever, is almost inconsequential. Its the handling of the corn problem that is important! (On my website, I do not distinguish between an allergy, sensitivty, or intolerance.) However, this is made difficult by the attitude of the medical community in America. Too many in the medical community will not take anything other than an actual positive classic allergy test seriously, even though there are varying degrees of food allergies, and certainly of the corn variety.
Additionally, the best, perhaps the only, treatment for a food allergy is to completely avoid the food. With corn, this includes all ingredients derived from it. This is difficult because the use of corn has grown over the last few decades, especially since the 1990's. Unfortunately, there isn't much that can be done about this now. The government has high sugar tariffs (in America) and subsidizes corn producers. Researchers are finding more and more ways to use corn, and the use of (unlabeled) genetically modified corn is as widespread as the use of corn itself.