I've had some bad experiences trying to return items, including electronics, in Japan. Definitely best to ask the store staff before purchasing. Most of the bigger electronics stores like Bic and Yodobashi have English speaking staff now, so you should be able to get a clear answer.

Or take rustykettle's advice and bring your laptop to the store to try with the display model. But, unless you have some really old laptop, I can't imagine why a new monitor would not be compatible with it... most connections are HDMI or digital now, so it should only be a matter of buying the right cable.