How do tell when my stylus is too much worn?

It is also debatable whether slightly rounded edges on a slightly worn fancy diamond, result in more audible record wear than say one with a simple conical tip. Of course the fancy diamond will sound better, but is there less record wear?

I have old records played many times on cheap old MM cartridges from the early 70’s that sound "mint" with expensive properly setup MC cartridges of today.

Records can survive more abuse than you think. Also your system resolution comes into the equation..

I feel the best solution is have two interchangeable arm tubes (like VPI and other brands) which contain a new and older cartridges for audible comparison.

If you can not hear a difference, you do not need a retip, or new cartridge.

I do that.

@rauliruegas , publishing sometimes do not match real facts.

Raul,
Do you listen to more vinyl than digital? Just curious how many times a year you re-tip? 

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.