Depends on what it is and how long the expiration date was. If the expiration date was rather shorts (days, weeks), then I generally just toss them.

Cans good or sterile medical goods, which last for years before it hits it's expiration date. I have no problem using them weeks or a few months after expiration, as long as the packaging is intact.

Ofcorse always use your nose and eyes. Smells or looks funny, than it goes straight in to the dumpster. Do note, that things do deteriate with time. So with food it might be taste. Medical supplies; sticky things start to stick less (or won't unstick from eachother in the package) and elastic things start become less elastic, etc. Which could also happen even before the expiration date when stored in crazy temperatures. So don't go years over the expiration date and think everything still works like new.