Obviously, it is quite difficult to answer this question briefly. But I will try
So most likely the first thing you will pay attention to is trader's profitability. But you should not base your choice only on this criterion. It's not right.
The next thing to look at is stability. If you realize that this trader makes some profit but does it stably, then most likely this is the most suitable choice.
Red flag is rapid growth of profits. If you see that trader's profitability increased with progression, it means that he is using risky strategies. For example, martingale.

Not a biggie, fortunately. Even if you doesn't have impressive expertise in the trading, everybody talks about how to choose a good trader to follow. You need to analyze each of the leader you consider as your primary one, check his history, examine such parameters as average profitability, look at the trading strategy and actually that's it. Pays attention to the number of followers too, as it's a direct indication of the trust to the leader from the community.