What you need to test magnets is a gaussmeter that measures the magnetic field. They come in a range of prices, and you can find some at Amazon.
It's possible to re-magnetize weak magnets with a magnetizer device usually known in slot car circles as a "magnet zapper". I haven't been involved with HO scale since my childhood, but as I recall, the T-Jet uses ceramic magnets. Mid-America recently introduced a magnetizer for ceramic magnets that might work with those old magnets to improve their strength. There are probably other sources. I've seen a variety of magnetizers for slot car magnets that mostly looked like a home-brew DIY project rather than a piece of commercial equipment. In my experience, the results weren't as good as buying new magnets, but since others seem to have had better results than I've experienced, it may well be influenced by the quality of the magnets, the quality and performance of the magnetizer, and the process used by the person operating the magnetizer. Where new magnets aren't a possibility, a magnetizer may provide a welcome improvement to an old stock of magnets.