The best thing I have found to do when it comes to balls is to buy a 10x Jewelers Loupe and look at each of them under magnification before installing them. I have found imperfections in some balls from every supplier I have tried so I just sort through balls as I install them and put the best balls in my better machines.
Remember NEVER handle balls with your bare hands your skin oil contains salt which can cause issues on balls rather quickly. Typically I grab balls out of the bag or package with a paper towel look each over then drop them into a machine never touching them with bare hands.
You will be surprised how many imperfections you will find on just about any ball when looking at it under 10x magnification. The nice thing about using a loupe to check balls is you can check used balls from time to time and decide when to replace them once you start seeing damage. Sometimes old balls in machines look better then the new ones I was getting ready to install. If that is the case I just wipe them down and drop them right back into the machine.
For under $10 you can grab a decent loupe off Amazon, eBay, etc. Cheaper than a set of balls and using it will save you money for years. The ones I use came from a local friend see ebay item number 401597281274 if you are curious what I am talking about.