The best way is to find someone who knows patent law in the jurisdiction you care about.
The general rule is 20 years from the filing date -- perhaps from the priority date. Most patent jurisdictions have a web site somewhere with their patent rules/laws posted. You can try to look it up yourself.
If the patent was filed more than 20 years ago, I'd say there's over a 90% chance that it's no longer in force. Of course, more information is always better.