The smell test is the best test. If it smells spoiled, it probably is. The average number of days the milk is still good after the expiration date is usally 3-4. I noticed that fat free or skim lasts longer than the regular high fat milk
In theory: Quite some time. In fact: Canned food becomes more and more tasteless and unappetizing as it ages. In practice: Discard canned food past it's expiration date.
This doesn't even deserve an answer, but not one day after it expires.
: It depends on the store where you shop. You need to ask, some stores take all expired coupons, some take up to 6 months after...
The date on margarine likely has more to do with quality and flavor rather than food safety. The date is probably more of a "best by" than and "expiration". It will be up to you whether to use it or...