You probably already know this, but;
If the fish seem ill or slightly off Always
test the water quality first.
If the water quality is 0ppm for Ammonia and Nitrite and under 40ppm for Nitrate, then you know it's something else.
Usually if I'm un sure on something, I just do a little research
Look for what other symptoms the fish should be displaying with the suspected illness and taking it from there.
It's good that you care so much about them
Sometimes with an illness you are un sure of, it is better to wait and see how things go
For example my goldfish Lucky, one day I noticed veins in her tail and I panicked. I thought she could have a bacterial infection, what if she's haemorhagging somewhere? Someone else mentioned it could be a sign impending fin rot.
The water quality was fine, so I knew it wasn't that and I kept a close eye on her, waiting for something to happen so I'd know what illness I was dealing with. And you know what? Nothing happened.
She wasn't ill. The veins in her tail never got worse, fin rot certainly never showed up and she never had any other symptoms to indicate a bacterial infection, she was perfectly happy and healthy, the veins weren't anything to worry about. I kept an eye on her for 20 days, just to make sure it was nothing serious.
So if she had of been ill, I would have known about it.
So sometimes if you are un sure if the fish is ill, as it seems to be acting healthy, it can be appropriate to wait and see. If the fish is ill, you will certainly find out and if it's nothing to worry about, you'll know that too