This is weird advice but whenever I test ride a new bike I try standing up while taking my hands off the handlebars. It takes a while to learn, but once you can its really easy to tell when there is some stability issue with the bike. Normally indicates something wasn't installed perfectly or has since come out of adjustment, but some bikes are just more stable than others. A stable bike will mostly correct itself even in gusts of wind, while some bikes will buck like a bronco. A shop owner taught me this