Holmes is assisted in his cases by the fact that he is not only an expert in crime, but also in many other things. Probably the most famous example of this is his ability to disguise himself "returned...in the character of an amiable Nonconformist clergyman" (A Scandal in Bohemia).
The characters of Sherlock Holmes include many different people and disguises, and he has to try very hard to not get caught being disguised as this can ruin the whole operation. But Sherlock Holmes himself wears a deer stalker hat which is his trademark. He comes across very smart and dignified. He also wears a cloak that goes over his shoulders with a very presentable tartan suit. Holmes speaks with a well educated voice and is very clear and does not mumble things up, unless he is disguised, he may use a different technique of talking. His most famous saying is “Elementary my Dear Watson” to his most faithful friend Dr Watson meaning to be positive.
Sherlock Holmes also treats Watson with a firm tone but is also endearing.
Sherlock Holmes’s methods of solving crimes involved the close study of a wide variety of subjects and the keeping of them and cross-referencing of good records, and would study a crime scene in great detail.
To solve crimes he has to find clues and to follow them, but he is also the kind of person who will think deeply about how he should go about doing these things.
Having accumulated all the information and evidence possible, he would form a theory about the crime and then test that theory, seek corroboration from his great friend and accomplice Dr Watson and collar the criminal.
He also uses a lot of unusual words when solving crimes, for example: "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."
There are many things about a person which will provide clues as to what that person's trade or profession is. If Sherlock Holmes knows where to look and what to look for, from these clues he can determine this person's trade or profession. However, to acquire this kind of ability he has to go through a period of training and after that to stay in form through constant practice.
In Sherlock Holmes’s time, everyone was wearing old fashioned clothing and would always or most of the time be very smart and well spoken. The streets were paved with cobbles and in one if the stories Sherlock Holmes famously tapped the pavement for a clue.
The population was also around 21 million in England and has changed a lot to now. We know what this was like in Sherlock Holmes’s time (1800s) because of the internet, books, paper etc and Researchers who can gather information by looking at the world and studying different areas around the world and many different locations.
Overall, I think the way Holmes can solve crimes is by analysing them, reasoning with them and also observing them. Also, he does not allow emotion to distract him from different clues. Holmes was a misogynist, so there was hardly any chance for emotion to enter his brain. I think that this quote best describes Sherlock Holmes "He was.... the most perfect reasoning machine that the world has ever seen" (A Scandal in Bohemia).
