I concur on the opaqueness/translucency. It will keep cooking for a while even once it is out of your pan, so bear that in mind too. Better under than over-cooked.
You can also go the route of getting a thin knife, popping it into the thickest part of the fish without going the whole way thru,just halfway, for a few seconds, then putting it onto your bottom lip. If it's cold, cook some more. If it's hot, it's certainly done. This applies to any fish, white or oily.
With Tuna and the like, you want to sear it just on the outside to get that sandwich look. Layer of white top and bottom, layer of pinish red in the middle, kind of the same appearance as a beetroot sandwhich. That is when to take it and plate it. Mind, this is only with really fresh fish.
Hope this helped.