Of course, if you got your eye on that guitar and you got the money. Ask to try it out.
It's not very nice to try things out totally, then tell the sales person you don't have the money.
But, it's ok to be honest. State that you just want to try out but don't have the cash now to get it. He'll probably let you try it anyway, but not pay too much attention on you.
If he refuse, just too bad, see if you really want to try & buy elsewhere.
Just some buying and selling ethics I guess, which is lacking a lot in Singapore in both consumer and retailer.