They check room keys on the way in. May let you in if rent a cabana for the day and you're not staying there.
They check for keys very closely. On our last trip in August we stayed one mid-week night at the Nugget with the rest comp'd at the Cal. One reason was to get a couple of room keys and access to the GN pool for the remainder of our stay. It worked out well.
They sell day passes. I believe it is $20. You can also buy a day pass to the spa, which gets you into the pool.
They will rent you daybeds and cabanas, which gets you in. Although im told they dont do that on busy weekends to non-hotel guests.
I could have sworn I have read lately that you could buy your way into the pool for $20 per person. Found it. http://www.lasvegasadvisor.com/coolpools.cfm?id=431 You might double check to make sure that is current information, though. However, that was good information if there are several of you. Buy a room there at least one night.