I have a custom bike that fits me like a glove but I still can't tell one frame from another at the LBS. I go on tours and sometimes rent bikes in cities I visit and I can never figure out what to ask for. I just do the old stand over test from childhood. Other people seem to be able to feel slight differences and specify exactly what they need. I wish I had a personal spec sheet that addressed how my custom bike specs match up against a variety of off the shelf bikes but that seems inordinately complex.