Check his body for sign of parasites like a gold dusting.
What do his gills look like do they look pale with excess slime or red and inflamed.
Does he looked bloated.
Does he act listless and lethagic.
Does he dart around the tank.
What it look like when he goes to the toilet.
Any red sores or streaking.
Any signs of flicking and rubbing.
is he still eating.
Any excess slime on his body.
Can he maintain his balance in the water.
Some more question here.
Breathing, opercules, gills
----Gasping, irregular opercular beat, rapid opercular beat (bacteria, dinoflagellates, parasites, toxins)
----Squirting water (parasitic dinoflagellates, gill parasites, gill infection)
----Opercules flared (gill parasites, gill infection, toxin damage to gills, or large gill parasite, as isopod)
----Mouth not closed (usually indicates gill/oxygen problem of some sort, heavy gill infection/infestation;
something caught in buccal cavity or throat = food, substrate, etc)
----Rapid or irregular opercular beat (bacteria, dinoflagellates [free-living or parasitic], toxins,
hyperplasia, parasites )
Info taken from here.http
/www.aquarticles.com/articles/management/Lawler_some_signs_of_sick_fish.html