How do you test lifters in a Chevy 350?

If you have any doubt, replace them. If the engine is running, you can pull a rocker cover and see if you can find a noisly lifter by slipping a thick feeler gauge between the valve and the rocker while the engine is running. If that significantly changes the sound of the noisy lifter, it's probably bad and you should replace it. Once the engine is torn down, just replace any lifter that's suspicious. Many people replace them every time they tear the engine down.