Just keep a close eye on your fish's color and behavior- don't worry about their size, that's too subjective and too long term, you might just have a runt.
Watch for their behavior- their fins should be upright, they should be active and eating (and swallowing, not spitting the food back out) and swimming level (not nose up or nose down). Any time you see clamped fins, hiding, fast gill movements, washed out color, hanging out at the surface in any of your fish- these are signs of fish stress.. and a key that you should probably do a large water change and bust out your test kits.
Once you get a little more experience you'll start to notice your fish acting wrong when something is wonky an have time to fix it before you have any fish deaths.