It depends, however it is said that most experienced GPs make the diagnosis in the majority of their consultations within about 2 minutes and before they stand up! It is a skill that they had little choice but to develop fairly early on,it is easily forgotten that until a few years ago most surgery appointments were 5 minutes. Allowing for getting in and getting out, this only left about 3 1/2 minutes to get the diagnosis! Perhaps it is reassuring to know that in about 80% of cases this diagnosis was correct, in about 15 to 18% the GP sent the patient away for tests to actually help make the diagnosis, rather than confirm it.
Most GPs frequently make telephone diagnoses and although the out of hours services SAY they don't make diagnoses, only risk assess, a lot of unseen diagnosis takes place there also.
GP for more years than I care to remember