This was good timing, since we just completed our first comprehensive update of the pool scene for the 2014 season. We say "first," because
Early on in the season, when the weather's still hit-or-miss and only the hardiest (or foolhardiest) are out lounging and swimming, a number of pools have relaxed admission policies, often letting anyone in gratis. But things have a habit of changing as the weather heats up, regular parties and concert series get underway, and space is at a premium.
Around spring break for the colleges, and definitely by Memorial Day, the more popular hotel-casino pools start excluding non-guests, or else charging a stiff cover (unless, of course, you're a nubile hottie or better yet, in a group of them, in which case it's pretty much open season all-summer long).
We should make the point that in general, pool-admission policy has become more and more liberal over the span of the past half-decade plus that we've been monitoring the scene closely.
There are a lot of competing party pools now -- Rehab, Wet Republic, Liquid, Bare, Daydream, Palms, Tao, Naked, Venus Pool, Marquee, Encore Beach Club, Daylight, The Voo, Sapphire Dayclub -- plus just more pool inventory in general, and it seems to be an instance of this working in the interests of the customer at least in terms of accessibility. (A few instances aside, there's no indication of any commensurate drop in cover charges where they apply, however, especially for guys, who might be charged up to $150 just to get in.)
A few welcome anyone to show up for free, usually during the week, but sometimes even on weekends. Some charge a nominal fee ($10-$15 for the day) for those staying at sister properties (Caesars Entertainment hotels, for example), and more for those who show up without an acceptable room key. Those that host major parties and events are open to all for a cover (usually $20 for women, $30 for men, but sometimes more).
Again, things can and do change overnight, so call ahead before you make a special journey somewhere (and if you can't speak to someone at the pool itself, ask for the spa, as they generally know more than the front desk or concierge, from our experience).