Fuses have datasheets, which have "maximum interrupting current" specs, as well as graphs of "how soon will it blow given X% over-current".
You need to consider what is downstream from the fuse, what the likely failure-mode of those components is, & thence how much fault current could flow. It could be as simple as a tantalum cap in the PSU that'll go nicely short-circuit, or it could be as fuzzy as something your circuit is powering off-board that, "for reasons", is drawing more than the expected current, particularly relevant if you're powering multiple such peripherals if there's potential for multiple simultaneous failures.
You don't really need to "prove" that the fuse will blow simply by applying some controlled resistance that will result in, say, 150% x maximum normal current, and confirm that the fuse does actually work as advertised - that's not much value.
You need to foresee what likely fault currents could be (by considering the characteristic fault modes of those components), and size the fuse accordingly.
You might also need to consider inrush current (e.g. charging caps at the moment of switch-on) to select the right style of fuse, & even what influence inrush current might have on the fuse's amp rating - you don't want a fuse so close to the normal operating current that switch-on inrush current is going to blow it.
If your fault currents are "high" (because some component has a propensity to go very low R when it fails), then the maximum interrupting capability of the fuse becomes relevant, especially for DC voltages >30V. 40+V is the ideal arc-welding voltage, and an arc can be sustained through the fuse even after it's blown. HRC, 'High Rupture Current' fuses are common here. But you're at 12V, so this is not so much an issue.
Yes, if you must, short out the fuse. But then the lawyers come for you...