You know one when you see one when it comes to "established fire-rings", but Generally:
- Complies with the objective rules.
- Has been used for multiple seasons, with evidence of a deep bed of soot from many uses
- Thoroughly built up with heavy rocks tall enough to block sparks and do not move when jostled
- Part of an established, and legal dispersed campsite. Although sometimes lunch spots will have a small ring.
They can be found throughout the park wherever flat spots for dispersed camping can be found, which often feels like every mile or two in Yosemite even in more remote parts of the park. However, some of these may be new and not "established", and in many cases one finds campfire rings in places that are not legal and these should be avoided.
As shown in another answer, the fire rings are typically built up of rocks, but metal rings are placed in very heavily used areas and campgrounds.
The rangers at the ranger station will be very explicit about fire regulations and current conditions/restrictions, it will be hard to feign ignorance.
Campfire rules in most of the Sierra Nevada wilderness areas and National Parks is exactly as cited. Established fire rings that are further than 100 Yards from Water (much further than you think), and below 9600 ft (elevation varies depending on management area) are Fair Game to use.
Additionally, during the summer, blanket fire bans can be imposed depending on wildfire conditions there has been a July->October fire ban in place for the last 10 years it seems.
I have on one occasion been approached by a passing wilderness ranger for using a campfire ring that was not "established", in that case it was a prominent campsite and ring, but much too close to the water. They requested that we put out our fire and move further away, noting that the campsite and fire ring was slated to be dismantled and posted due to its heavy use. We complied and had no issues or fines, as long as you act in good faith I would expect the same. However, campfire during a fire ban period is bad juju and will incur the wrath of rangers.
I love campfires, so do not begrudge you wanting to make a fire, but in places heavily impacted like May Lake and other parts of Yosemite, I attempt to abstain because the evidence of human impact is everywhere. I try to save it for the more remote areas, where human impact and human travel is sparse.
However, Please note. Higher elevation, above the tree line in areas that are not wildfire ecosystems and with sparse trees, a single largish campfire can burn years of dead-fall that would have served as food for various bugs and critters, sustaining small birds and animals like frogs and lizards and recycling those nutrients into the soil for the delicate plants that are then eaten by other small animals like the cozy Pika. If you find yourself looking at a desolate landscape, even below the legal elevation, try to abstain from picking the few sparse twigs available for you to build a fire.