Cooked shrimp or fish will look opaque on the outside, instead of translucent.  As far as if fish ceviche is cooked long enough, that is personal preference.  Some prefer the seafood on the softer, less cooked side, while others prefer it thoroughly cooked.  Keep in mind, if you taste the ceviche and it is too “raw” for your liking – then continue to marinate. Ceviche de camaron should always be cooked completely through.