Detect Balance is an unoffical, widely accepted tool to evaluate the power of homebrew races
The tool has the form of an online spreadsheet that you can access and copy freely. It was originally launched in 2016 on Reddit by user u/jwbjerk, also going by the name of Eleazar, and is based on an earlier such tool by James Muscius.
The way this tool works is that it assigns a positive or negative point value to nearly all the features that appear in published player races, and then measures the strength of a given race by summing up the point value.
For example, a +2 Ability Score increase is worth 8 points, darkvision is worth 3 points, and a walking speed of 25 is worth -2 points.
The tool has been evaluated by scoring published races, and recommends
The average score for PHB and EE races by this scale is about 25.
The recommended score for new homebrew races is 24 to 27. Races should generally not eclipse 30 or fall below 20.
The tool has been maintained and updated based on feedback from the community over several years, and is heavily used on this site to evaluate homebrew races.
Weaknesses of the approach
There are disadvantages to this point-scoring approach, mainly in that features are not independent and can reinforce or weaken each other.
For example the ability to ignore a common type of Rough terrain (worth 2 points) is a lot less valuable if you also have the ability to fly (worth 16 points) as you can just fly over the terrain. Some abilities also are strong for some classes, but weak for others, for example the ability to not be slowed by armor (worth 2 points) does nothing for a wizard, but might be great for a fighter.
Another serious flaw is that negative features and positive features numerically appear to balance each other out, but multiple unbalancing features do not create a balanced race. You can see a great example of that in action here. So you should treat any race that uses negatively valued flaws to offset powerful features and stay within range with suspicion.
Quadratic Wizard gives an examples of how you can combine features in a way that is unbalanced for a given class, and historic precedent for point-buy systems being exploited in character creation by optimizing players. If you only used Detect Balance to judge fairness, a designer could combine features that numerically appear balanced, but are not.
Benefits of the approach
However, there are also benefits for using the tool. There is a reason why it has been used so widely.
First, it makes it much easier to score the comparative power of various features, if looked at in a vacuum. For example, what is stronger, a +2 to your pick of an Ability Score increase (which is generally seen to be better than most feats), or a free pick of a feat at first level? Normally you would need to have a long argument and discussion about it.
Because Detect Balance has been so well vetted and discussed already, most people accept that the values there are robust. If you use Detect Balance, you can just look up that the +2 is worth 8 points, and the free feat at first level is worth 20 points, so the feat is a lot stronger.
I was actually surprised how well calibrated it is. I had a question about how the tool concluded that darkvision is as valuable as a free skill pick (3 points each), and there was an official feat that gave this exact equivalence to support it.
Second, it provides a structured way to evaluate races. You go over each feature, and assign Detect Balance points to each of the ones that are standard, and ballpark points to those that are not by comparing them to the closest standard ones. Then you can layer synergy aspects, or special strength for certain classes on top to come to an overall evaluation of the class.