Try out the lenses and try out a brand new lens coming out of the box are two different things. If the store has demo lenses, it is quite ok to ask them to let you try it out without any obligations. However, if you ask them to open up a new box and try out the lens, that means you intend to buy the lens unless there is something wrong with that particular lens.
Some of the electronic chain stores have quite a few popular lenses on demo so there is no problem testing out those since they are not brand new lenses. For Canon (not sure about Nikon or Sony), you can go to Vivo city showroom to try the lens out before making your HK trip, you might not have enough time to try out the lenses when you are traveling.