When Dr. Watson walks into Sherlock Holmes' apartment, Holmes proclaims that the good doctor has put on at least seven and a half pounds since his wedding.
He also tells Dr. Watson that he (Dr. Watson) has gotten wet lately and that the doctor's servant girl is a clumsy and careless housemaid. Additionally, Holmes also observes that Dr. Watson has been busy in his medical practice.
In order to deduce that Dr. Watson has gotten wet lately, Holmes looks at the doctor's shoes. His left shoe shows six almost parallel cuts in the leather, which appears to have been the work of Dr. Watson's careless 'London slavey.' A slavey is an English servant-maid responsible for the menial tasks in a household. Holmes deduces that the servant girl's rough scraping of the soles (in an effort to rid the shoe of traces of mud) has produced the almost imperceptible cuts in the leather. From the cuts in the sole, Holmes deduces that only a very clumsy and negligent servant girl could be responsible for such a mishap.
As for Dr. Watson being active in his profession, Holmes assures the doctor that he could smell the evidence of his practice from the instance he walked into Holmes' apartment. Accordingly, Dr. Watson exudes the smell of iodoform, an antiseptic commonly used in hospitals at the time, and his right fore-finger betrays a silver nitrate stain. In the medical profession, silver nitrate is used for cauterizing wounds and killing unhealthy tissue. Holmes also observes that there is a bulge on the right side of Dr. Watson's top-hat, where he has put his stethoscope.