Which ball joint is it

I know that the ball joints are bad in my car and need to be replaced. The suspension/steering makes a creaky, squeaky noise - kind of like squeaky door hinges or an old mattress spring when steering or going over rough roads.

I have an auto shop to fix it that lets me bring my own parts and then I just pay them for the labor. My question is, how do I know if it is the upper or the lower ball joints that are bad so that I can buy the correct parts?