Take your proposition. Put your closest competitor's name on it. Read it aloud.
If it still works, it was never yours.
That is the whole test, and it takes about four seconds, which is roughly four seconds longer than most propositions have ever been examined. I have watched propositions survive six months of development, three rounds of research and a board sign-off, and then fail this in one reading. Not because the people involved were careless. Because nobody ever held it up against the one thing that matters, which is whether someone else could say it.
The reason it works is that it separates two things that feel identical from the inside. True and distinctive.
Almost every failed proposition is true. That is the trap. The business really does care about its customers, really does deliver reliably, really does combine expertise with agility. Everyone in the room can evidence it. So it passes internal scrutiny, because internal scrutiny asks "is this accurate" and the answer is yes. What internal scrutiny almost never asks is "is this available to anyone else", and the answer to that is also yes, which is the problem.
The rival test forces the second question by making it physical. You have to say the competitor's name. You have to hear it fit.
Three notes on running it properly.
Use your nearest rival, not your worst one. The comparison that flatters you is worthless. Pick the one who beats you in pitches, the one whose name comes up in the room when you are not there. If your proposition survives against them, it will survive against the rest.
Run it with sales, not with marketing. Marketing has spent months with the words and has lost the ability to hear them cold. Sales people can tell you in about a second whether a line would get them anywhere, because they have to say things out loud to strangers for a living. Their judgement on this is better than the board's.
Do not fix a failure by adding. When a proposition fails the test, the reflex is to bolt on a qualifier that makes it more specifically true of you. "Reliable delivery, backed by thirty years of engineering heritage." Now it is longer, and still nobody will remember it. Failure means the underlying idea is category-wide, and you need a different idea, not a longer sentence.
There is a harder second stage, once you have something that passes. Put a competitor's name on it and check it is now wrong. Not merely ill-fitting. Actually false when applied to them. That is the difference between a proposition that distinguishes you and a proposition that positions them, which is the more valuable of the two.