How do I know when my computer fried in my car

Hello, a "computer being fried" is not a very accurate technical description of what is going on with your vehicle in the first place. It is true that moisture can cause internal damage to a processor. Care needs to be given to resolve the leak issue so that any repairs can be successful for a longer term. Moisture in any control unit or processor can and will damage various electrical contacts and circuit boards, creating a lack of functionality within whatever the unit is. It may seem as if "everything still works" -- when in fact certain functions are compromised. The key not turning is likely due to an electrical function that occurs when the key is detected in the ignition key cylinder. I would say it is very important to fix things that are known to be faulty before trying to reason or rationalize the meaning or potential hypothetical consequences of what has been seen to be detrimental. Water + electronics is never good. Fix this then recheck for further issues.