worn crank rings
 

How can you tell when the crank rings are worn to the point that they need to be replaced? Also there are some teeth that are missing the tips of them, is that anything to be concerned with?

The crankset is a Shimano FC-M521 from what I could find online.

The missing tips might be designed that way. Teeth on worn cranks will start to resemble shark fins, sort or sharp with a rearward curve.

shifting aids

I replace my chainring teeth when they get very sharp & pointy (except the ones that are designed with a lower profile to begin with). If you look at a new chainring you will notice that all the teeth will have at least a small flat top on them. This flat top will totally disappear. Also the U shaped bottom part of the teeth will have a flat spot. Making the U look wider than it was when it was new.

As mentioned earlier -- shark tooth shaped chainring teeth is another way of telling if it is time to change the chainring. Shark tooth shaped chainring is bad and new ring needs to be installed.

Chainrings last longer when you replace your chain when it get out of spec.
aka 'stretched'..

There's no logic to replacing chainrings before you have to. They're worn when it's bad enough that the chain slips under load. For grannys and middle rings in mtb that can happen pretty quickly, but road rings last amazingly long.

The rings on my older road bike have over 50,000 miles on them. The teeth are sharper than a ninja star, and I could probably slice salami with them, but they still run as smooth as silk.

BTW- the cut down or "chipped" teeth were probably like that when the rings were new. Those are shifting gates and part of the hyperglide shifting system.