Ok, here's the deal on cardiac caths:
Cardiac catheterization is a procedure that is done to evaluate the blood vessels of the heart (and occasionally the vessels leading to and from the heart)
As such, they are useful in identifying problems with those vesssels - narrowing usually due to cholesterol deposits.
Ryan Shay had "fibrotic" tissue in his heart. Fibrosis is a fancy medical term for scar tissue, but the key is that the fibrosis was not in the blood vessels. It was found in the actual working part of the heart - presumably the atria and the ventricles. Shay's problem had absolutely nothing to do with the blood vessels of the heart (the coronary arteries and their branches) or the vessels leading to or from the heart.
The only test that can identify fibrotic tissue of the heart is a biopsy - where they stick a hollow needle into the heart and extract tissue - or an autopsy. Heart biopsies are extremely dangerous, and basically used as a test of last resort when a patient is on the verge of death.
From all media reports, Ryan and his family were aware that there was something abnormal with his heart, dating back at least until the time he was 14 years old. From the same media accounts, it appears Ryan was followed by cardiologists ( = heart doctors) over the years.
What killed Shay was a spontaneous abnormal electrical rhythm (i.e. an arrhythmia). These abnormal rhythms were presumably caused by the enlargement of the heart over the years.
Think of it this way - in a very simplified model:
Running throughout the heart there are many wires that conduct electrical charges. These charges are synchronized so that the entire heart beats in unison. When the heart gets bigger than it should, some of those wires stretch and bend. Thus, some electrical charges are conducted normally, some take longer due to the stretching. Some might even begin to "fire" on their own, conducting electricity out of seeminly nowhere.
When that happens, it's just a matter of time before they pass through the entire heart, and make it impossible for the heart to beat normally, and thus pump blood normally.
The only "cure" at that point is defibrillation ( = shocking with the paddles) and sometimes that is not enough to reset the rhythm.
There are very few tests to a spontaneous abnormal rhythm. An electrocardiogram (EKG) can measure the electrical impulses of the heart, but unless there is a problem right then and there, it won't show up. Longer term electrical monitoring (called a Holter or event monitor) can be used, but again, unless an event happens when the person is wearing the monitor, you won't find the problem.
Finding a spontaneous abnormal rhythm is akin to standing a random street corner waiting for an accident to happen. The law of statistics tell you that is possible, but you can stand there for a day, a week, a month, or a year and not see an accident. And the day you leave there could be a 5 car pile up. It's completely random, hit or miss, and at this time there is no good way to predict it.
Hope this helps.