Sorry to hear about your loss
Internal parasites don't seem to just cause fish to just keel over and die.. Generally internal parasites cause the fish to get run down, they will gradually loose colour, loose appetite, start fading away, just be lethargic, have white stringy poo's, red worms out bum etc. Of course there are exceptions to this but I would expect you to have seen something going wrong.
The only way to tell for sure is to do an autopsy, the problem is knowing what you are doing and knowing what your actually looking at and what it should look like, so unless its something obvious it probably wont achieve anything.
The most annoying thing with fish is not knowing what killed them sometimes they just die with no warning, keep an eye on your other fish and if you see any problems that would suggest parasites then deal with it then.