Well, for starters, a good rule of thumb is that goldfish should have about 75l each. Since goldfish are very messy, it's good to do this for starters, even if they are considered small at the moment. As for signs to watch out for, I've read that goldfish are prone to constipation, but I'm not sure about anything else (I'll leave that to the goldfish experts here!). So for now, I'd just be on the lookout for abnormal behavior (compared to what they are like now) such as different swimming patterns, feces (or lack thereof), and eating habits. It's a good idea to at least change the water once a week in their current tank.