If you are honestly open to buying a good cable if it works, any good dealer will loan you trial cables too. It would be dishonest if you were unwilling to buy though.
For me it is a cart & horse situation. I would need to believe there would be a difference before I would begin the discussions with a dealer. At this point the conversation would be;
Me: I would like to try some speaker cables in my system.
Dealer: Which cables would you like to try?
Me: The cables which would provide the most noticeable difference.
Dealer: Well I have the Quasimoto MK VIII with Nanominium wire and Vibranium connectors all cryogenically treated creating a single crystal matrix. These are on sale today for $2,375 per meter.
Me: Uh....
Not being a dishonest person I would never waste a dealer's time with no intention to buy something. This is one of the reasons I find it difficult to venture into HiFi stores.