Reddit user manbeef says:
"Hit up a couple of grocery stores in your area or look at flyers. Compare your food staples and see which store is cheaper. Hopefully you'll notice a trend. In my area, I have three grocery stores to choose from: one is overall a lot more expensive, but has a lot of unique and hard-to-find items; the second is expensive, but has a good selection of produce and meats; the third one is cheaper overall, but lacks some items. I usually shop on my way home from work, hitting the cheap store first, getting everything I can, then hitting the midpriced store the next day on my way home for the odd item I can't get.
"What you should be able to do is walk into a store, look at a product, and know what a good price for that product is. If it's a deal, buy it, or buy some extra. I still get caught buying something at one store when I don't know the average price, then realizing at the next store that it's half the price there."