Get a set of visually-similar fuses that claim to be the same rating. (Visually because you need the shape to be the same). When testing a few to destruction, record the temperature of the fusing element and resistance and power over time. Also measure the temperature at operating current.

Now the fuse you want to have the utmost trust in, test it at at half the time it should fail for a given i^2t. The temperature, resistance, and power curve should match the ones you tested destructively. The temperature at operating current should also be similar. If so, you can expect it to fail the way your destructively-tested fuses did.

Now, if you trust the metal composition and fusing element shape, a simple resistance meter will tell you if two fuses will behave the same.