I've got to agree with the majority here. The OPs assertion seems to be that the best way to teach is by letting people make mistakes. While that may be a good way to *learn*, it is never a good way to teach. Especially when someones heath or life is on the line.
In any business or education environment, intentionally exposing someone to risk - even if well intentioned - is going to be good grounds for dismissal, maybe even getting sued. The limit for safe handling without special procedures is generally considered to be around 50V, and there are exceptions even to that. I'm am personally especially careful with anything above 30V, or where high currents are possible.
If you want to measure mains voltage, use a multimeter & probes appropriate for the task (i.e. CAT II or better).
If you want to check mains waveform, you can use a isolation transformer - preferably one with a lower secondary voltage, like the doorbel transformer discussed above. Or you could use an oscilloscope designed for power electronics use - typically battery powered for isolation, and with a high voltage probe.
If you want to check a fuse, remove it and use a mutlimeter in ohms mode - and never do it with power on. In fact on some older equipment, it can be dangerous to remove a fuse while powered due to poor fuse holder design. I still occasionally see low voltage fuse holders used on new mains equipment.
Yes, there are ways to do all of these in the non-ideal ways suggested above - and you may even do that in your home environment where you take that risk on yourself, but you don't go doing that in a workplace, or worse teaching others to do it. By doing so, you are placing both your job and others at risk. If you don't agree, please chat with your boss about risk management training.
And yes, I've had some minor 240V shocks in my life - but only as a teenager when I really should have known better. And now I do!