Symptoms of internal bugs (should have used the word bacteria rather than parasite I think) are a hollowing of the belly, red blood streaks under the surface, gills pale and eroded, loss of appetite, listlessness and a darkening of colour.
Most common parasite is white spot of course. The fish will have rapid gill movement, flick and rub against objects, then break out in tiny white spots (so they look like they have been sprinkled with salt).
External parasite symptoms include the flicking and rubbing, rapid gill movement and red, irritated areas of skin.
I do not know a lot about fish diseases as, touch wood, I have not had any on my tanks for years. I have been looking at web sites for info