When I started cycling, I couldn't understand how my LBS man could look me up-and-down and tell me my frame size and stem length.
Over the years and being in and out of bike shops and frame builders, I realise those old guys have had forty or fifty years of sizing bikes to people.
Now, I have forty years of bike experience and when I get asked by a 'newbie' at work who wants to buy a bike, I can do the 'looksie' and tell him the framesize.
I haven't been punched in the gob yet, so I guess they were all happy with my recommendations.
But it comes down to the fact that to be totally reassured, a Framebuilder is the only man who can help.