You're probably OK.
A quarantine system is for addicts like me who have a herd of expensive fish who are overly paranoid.
If you are beginning in the hobby and take care when buying you should be OK. My general guidelines are to look in the tanks to make sure there aren't any sick, dying, or dead fish in the tanks with them at the store. Can you go when they're feeding? If so, are the fish you're looking at eating n a healthy manner?
There are other things to look for and i'm sure i'm missing something that one of the big brain folk around here will catch me on.
An example for me. I got some peacock gudgeons to add to my 100 gallon planted tank which already has a large group of Cardinal tetras and some other fish in it. I would rather keep those gudgeons in a holding tank for a week or two while I make sure one doesnt develop dropsy, ich, or something else.
As for telling the signs of disease, I believe this site has some documentation on it about diseases and diagnosing what might be wrong with a given fish.