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Context : while playing against a complete stranger that hide its rank/level.
I feel like it would be possible to guess the opponent's strength by analyzing its responses to tesujis, probes, etc. during the game.
Is there some patterns (or moves or errors) that indicates that a player has or has not reach a certain plateau ?
If it's the case, is there any on-line resource on the subject ?
Probably the best way to gauge a player's strength (aside from playing against them of course) is to present them with Tsumego of varying difficulties. From Sensei's Library:
Tsumego, a Japanese go term adopted into English, are problems mainly about life and death, but also about ko, capturing races, cutting, connecting, etc. As a rule they are local problems, but a few involve the whole board.
There are many published collections of tsumego, or life-and-death problems, many in Asian languages but increasingly many in Western languages. Many collections are available on the internet. SL also has a growing collection of go problems.
Most professional players and top amateur players agree that solving tsumego is the best way to improve.
Sensei's Library has a plethora of tsumego from a variety of sources.
Personally, I've also used GoProblems.com a lot to improve my own skills. I like that they try to associate individual tsumego with what rank should be able to solve them. They also have an incredible number to choose from. As of this posting they claim to have 11,196 problems.
In addition to the way proposed by Nathan, I would like to add a few further ideas.
One simple way to judge the relative strength of two players is to look at their mutual game history (if it is large enough, of course!) and consider the percentage of won and lost games. For instance the EGF rating system has this property:
This setting gives about 30% probability for beating a 1 grade stronger opponent.
So if one players wins only 30% of the games, we may assume he is one stone weaker. Other percentages can be translated via their formula.
Of course, a large number of games between the two players is required. Also, relative strength is not transitive: It may well be that player A usually beats B, B usually beats C, and yet C beats A more often than not.
Historically, the average score difference in (sufficiently many) even games is a good indicator.
One stone difference in strength equals two komi, and komi is usually around 6-7 points. So, for instance, if one players wins by around 26 points on average, we may assume he is 2 stones stronger.
Obviously, this approach suffers from resignations - the players would need to continue lost games until scoring. simply applying a threshold seems to be impossible in general, and may only be useful in specific instances.
I was interested in seeing how accurately we can judge the level of Go players from their moves and created a website for this purpose: https://kyudan.net
It displays the first 100 moves of an anonymized game. Once you submit your estimation of the respective level of the players, their real rank is displayed. All players are dan level up (so the options rank from 1 dan to 9 dan only). More information about the process are described here.
When I last looked, there were more than 2000 results submitted by more than 100 people. There was a significant (from appearance, I did not really number crunch this) difference in estimation accuracy between different judges. The most accurate people were often within 1 - 2 stones of the real ranks, but on average people were off by 2 - 3 stones. You can see my personal results here.
Considering that simply always selecting "5 dan" will lead to being within 4 stones of the correct answer, it seems apparent that judging player level from a single game is not easy for most of us.
