In addition to the good reply from @user87317:

Before making a decision, I prepare a fully-updated Live USB with my favourite applications. I pop into the store and, with permission from the staff, boot the display model using my Live USB.

The laptop needs to successfully boot and pass various tests, e.g. camera, microphone, playing multimedia, wireless Internet, touch-screen, connect to my phone via USB and Bluetooth (if it has Bluetooth), and run my must-have applications. (Test this on your existing computer to iron out any problems before you get to the store.)

The staff member is usually keen to give authorisation, because he hasn't seen Ubuntu before, is curious as to how you can boot something other than Windows from a Windows computer, and knows that he's more likely to make a sale if he allows you to proceed.