I agree with most everyone here. It depends on what it is, and how far out of date it is.

Here in Canada, we have a "Best Before" date. That means it should be safe to eat, and also still be at its best, until that date.

Meat and dairy items, or items containing either, cannot be trusted past that date. No preservative can keep food safe forever. You're taking a chance on becoming very sick if you attempt it.

Even some vegetables and fruits can fall prey to botulism, which is odourless and colourless. So I stick by the "Best By" dates, in 99.9% of circumstances.

Otherwise (and this doesn't happen often), I stick to the taste and smell test. The slightest whiff of stale, or "off", and it goes out.