A failed launch, as in an initially successful launch that went wrong in the air, can afterwards be spotted even on commercial satellite images: https://arstechnica.com/space/2024/09/satellite-images-suggest-test-of-russian-super-weapon-failed-spectacularly/ The usa and nato probably know long before those amateur spotters do.

If the rocket fails to launch at all when the button is pressed, then noone will be allowed to know probably. It could be that they tried to launch 10 and only 1 ignited, or maybe there was just the one. Russia isn't going to tell the truth about anything so it's anyone's guess. If it fails to ignite, then I'd expect them to just pack up the rocket again and continue to pretend doing maintenance and have soldiers guarding the stuff.