For example, my husband came home from the grocery store and said he saw a buy-one-get-one-free deal on London Broil. It was $6.99 per pound. Some people would see the BOGO offer and stock up, thinking that buy-one-get-one-free is automatically a good deal. My husband didn't buy it because he knew better. $6.99 a pound isn't a good price for London Broil. Even $3.50 - the BOGO price - isn't that great. For the record, I usually buy London Broil when its being sold for under $3 a pound. I buy ground beef or boneless, skinless chicken breasts when they're $1.99 a pound or less.
So how do you know what things cost, so you can tell the difference between a good deal, a great deal, and no deal at all? For starters, know that "Special Buys!" "BOGO Deals!" and "Stock-Up Sales!" aren't always the amazing bargains they claim to be. Those catch phrases are designed to get you through the door, and the grocer hopes they'll be enough to get you to stock up, even when the items are being sold at the regular price.
You can get a basic idea of the going rate or certain things by doing a quick Google search, but know that prices can vary significantly by region, and that grocery prices fluctuate in time.
Make a grocery price list. If your store has a website, you might be able to find pricing online. If not, you'll have to take a trip to the store. Take a notebook, and record the items you buy regularly, along with their prices. Write down the name, location in the store, and the price. If it's on sale, record both the regular and sale price, and if you can, write down the unit price (price per pound, per ounce, per 100, etc.).
Take this notebook with you whenever you shop, and note price changes if and when they occur.
Watch store circulars for things to go on sale, and compare the sale prices to the regular prices you recorded. You should also add these sale prices to your notebook so you'll know for next time, especially in cases where sale prices vary.
If you have freezer room to stock up when things go on sale, do it. That way when you need chicken for a recipe you can grab it from your freezer, instead of running to the store and paying whatever today's price is.