Ball joint failed in six months - how do you know if a new one is good?

1. Lots of things can cause accelerated wear of ball joints, including balancing issues, bent rims, etc. So check all that stuff out first.
2. Depending on what model car it is, you may want to avoid the chain stores altogether and get an OEM part.
3. If it doesn't come pre-greased, grease it with the red Mobil 1 stuff first. Even if it does come pre-greased, my inclination is to clean it and regrease it before installing.

What does the bad one look like? Wear is even or not?

--scott

Buy genuine OEM and fit it correctly.