In addition to Peto's answer (he's correct by saying you can't).
So as a customer , how can we know the actual price of any object through its bar code without having bar code Scanner?
- The price of an item is what's encoded in the store system. So the way to know an item price (if it's not written near the item) is to let the cashier scan it.
I assume that the price is written near the item, or with a sticker on it - if so, you can (and should, if you care about your money) compare the receipt to what you expected to pay.
There are errors, it's inevitable - it might be in the encoding process in the vendor system, or in the cashier typing (for products without a barcode on them, such as fruits for example).
Anyway, as a wise consumer, it's your responsibility to go over your receipt and check that your payment was correct - if not, go back to the cashier to fix that and get your money back.
And as for what BrettFromLA said:
"Usually customer is overcharged." I think you need to go to stores that are more trustworthy!
Errors may (and will) occur, but not dramatically and certainly not usually.