How do tell when my stylus is too much worn?

Of course I should remove the cartridge and view under a microscope but un mounting and remounting is a perilous business that I would like to avoid.

Looks like you will void visual examining of stylus and only limit yourself to finding stylus problems only by ear which is somewhat inefficient.
Very often you may find degraded performance of inner tracks including sibilance. Also in diamond stylus less likely diamond tip is affected, but more likely stylus mount and cantilever which is visible under jewelers loupe 

Beogram turntables with moving coil are still around since 80's and sounding good. Most of decks with lost styluses had been mishandled by owner. Others perform like champions. Unfortunately for some reasons cartridges aren't made anymore for them, or worth the fortune. I hoarded at least several Beograms with needles and checked their styluses and noticed little to no gunk and no loss of suspension integrity that is including tip and cantilever mounts. 
To be sure I carefully applied CRC spray on each and every one of the vintage cartridges. The playback is exceptionally clean for outer and inner tracks. 
Diamond tip can serve forever, but suspension and cantilever is what wears out. In case of beograms with lightest arms, the suspension is never too stressed. 

@rauliruegas , publishing sometimes do not match real facts.

another one of signs of warn stylus is it's not erect anymore and may fall down almost to the chassis under normal tracking weight.