Need Some Help At Sephora?

I shop at Sephora and its a pain to have to return something that is the wrong shade and my friend just told me that I can ask for samples but I am not sure how to say it. I really want to try the Nars Sheer Glow Foundation and the Laura Mercier Silk Creme Foundation but I cant afford to try them both any ideas on how to ask for samples for both? Would it be better if I buy something?