It may very well be infringement, because you are describing a "use in commerce" of the trademark, even though you are not selling the samples. This is especially dangerous if you intend to let anyone keep the samples: Once these samples are out of your hands, you have no idea what will be done with them, and anyone looking at them can be expected to assume that the logos are authorized to be used on these products. Even if you only use these mockups in sales presentations, confusion is still possible -- maybe even "likely." Without knowing more, you would probably not want to proceed with this approach without sitting down with an attorney who could advise you regarding the specifics.
My colleague Mr. Coleman is right on the money but too kind: YES it would be unlawful to affix another company's trade name or any of its trademarks onto a product that the company does not sell.
The fact that you intend to show your product (branded as if some other company is the source of the product) only to "potential" customers is irrelevant. EVERYONE in business shows their products to potential customers -- that's called advertising.
However, if your "potential customer" universe consists of three folks in a conference room (or some other very, very limited and controlled environment) who may, or may not, provide your company with funding or shelf space or something else of value and your product display is a mock up used solely for demonstration purposes that's another matter -- and very likely lawful so long as you maintain physical control of the mock up. Beyond that limited environment, however, you may not tread.
The inevitable follow up question that is often asked, given the sage advice of the other posters is something along the lines of "can we still use the logo on the mockups if we clearly indicate that the mockups are simply samples that are not for sale?" While that approach might steer you away from infringement to an extent, it is still within the ambit of other trademark doctrines that the actual rights holder may use against you. Moreover, the specific manner and nature of the "disclaimers" would have to be sufficient to prevent a likelihood of confusion in the public's mind regarding the source of the goods. That being said, I would suggest not only using a disclaimer as descirbed here, but also use a mockup logo that is slightly evocative of the actual logo in mind, but is far enough removed to be clear that you are not tryng to actively purport an affiliation between our product and the rights holder at this time. As with previous advice, I think you should consult with a business attorney that is knowlegable about trademark law.
I hope this helps.
Disclaimer: This answer is for informational purposes only and does not constitute general or specific legal advice, nor create an attorney client relationship.