just asking about front.
how do i know if i have a bad one?
cheaper to just get it done ?
just fix a bad one, upper and lower on that side, or upper and lower on both sides ?
cost to get whatever you suggest done?
Jack up one side at a time by the axle. Use something that will get you some leverage and put it under one front wheel and try to pry the wheel up at the outer edge of the tire. Have a second person watch closely at the ball joints and watch for slop,movement or whatever you want to call it. Pay attention to whats moving if anything as this will also reveal any slop in the wheel bearing units and you will be able to distinguish whats worn.
Far as replacing what? If you have one bad the others are not longing for replacement as well. Depending on mileage of your rig you may wanna replace the front axle joints as well while its torn down. Like they say, theres never enough time to do it right, but always enough time to fix it a second time.
I always replace in sets (upper and lower)..if nobody has used knurled ball joints then spicer is the only one I will use.
You can buy the kit from harbor freight or probably rent from autozone etc
They arent difficult to do but they only go in one way so make sure you are removing and installing them correctly. There is lots of youtube stuff on this any dana 30 that's less than 20 years old is probably going to be the same process