1-make sure each item's cost matches one by one as the cashier enters them.
2-Check that the total count of items matches, take your time .
3-make sure all what you buy goes into the plastic bags you are taking away with you!
4-Check that you receive the correct change.
5-If you sense something is not right ask politely for explanation, dont be shy the cashier is there to serve you .
make this a habbit.