A combination of a fitting (which I last did as a “fitting” around 20 years ago), and knowing what I wanted position wise.
Last TT bike I bought (a used P2k in 2018) I figured out my position with a couple of cheap stems and some time on the trainer,
Really, unless I can get the dealer to swap stems and bars, and adjust everything to exactly where I want it, which might take me a week to figure out on a road bike, I’m not gonna tell much from a test ride. I mostly just care about the contact points anyway, which are adjustable on any bike. Road bikes take me a while, and mean removing bar tape, swapping stems, rotating bars and repositioning levers, swapping saddles, etc etc.
Swimming Workout of the Day: Favourite Swim Sets: 2020 National Masters Champion - M50-54 - 50m Butterfly