Choosing generic products when quality differences are hard to judge

For pantry staples I'll happily buy the store brand. For things like batteries, dish soap or shoes, the cheapest choice sometimes lasts half as long, so the label price is misleading. I keep falling into the trap of either assuming generic is always equal or assuming brand name must be better. How do you test quality differences without buying five versions of everything? I'm leaning on cost-per-use and a short list of exceptions that have actually disappointed me.