foolproof

[ fool-proof ]

/ ˈfulˌpruf /

involving no risk or harm, even when tampered with.

never-failing: a foolproof method.

Origin of foolproof

An Americanism dating back to 1900–05; fool¹ + -proof

A “foolproof” way to make the diagnosis would be news indeed!

How to Tell When a Scientific Study Is Total B.S. |Russell Saunders |August 22, 2014 |DAILY BEAST
Cry No More Every parent tires of hearing their child cry, but not everyone finds such a foolproof way to make it stop.

North West Debuts, Rob Ford Arm-Wrestles Hulk Hogan & More Viral Videos |The Daily Beast Video |August 24, 2013 |DAILY BEAST
Of all the potential holiday gifts, few are as foolproof as cash or gift cards.

What to Do With Unwanted Gift Cards |Nina Strochlic |December 26, 2012 |DAILY BEAST
Dan Gross on why the episode is a reminder that the vaunted cloud is much like the Web in the 1990s—young and far from foolproof.

Blame It on the Cloud: Amazon Outage Shows Peril of Outsourcing Data |Daniel Gross |October 25, 2012 |DAILY BEAST

The only foolproof way to avoid pregnancy is, of course, not to have sex.

Gail Collins on Texas’s Abstinence Sex Education Problems |Gail Collins |June 4, 2012 |DAILY BEAST
No matter how foolproof you think your setup is, if you start smuggling you're bound to get caught.

Smugglers' Reef |John Blaine
But trying to work out a foolproof plan would be a waste of time when she didn't know just what she was up against.

Legacy |James H Schmitz
This time he wasn't afraid of meeting a Bandersnatch or anything else, for he had a method of escape that was foolproof.

Insidekick |Jesse Franklin Bone
He tried to figure out some foolproof way of cabling to Havana, but the censorship hazards were too great.

The Five Arrows |Allan Chase
Fastest lens, foolproof operation, guaranteed for the life of the owner, and retailing for exactly twenty-five dollars.

Revolution |Dallas McCord Reynolds

foolproof

/ (ˈfuːlˌpruːf) /

proof against failure; infallible a foolproof idea

(esp of machines) proof against human misuse, error, etc