You can access this through the elevator inside the casino. The first possible access on the roof, one of several that were placed next to the helipad. The devs seem to want people to access the casino from the roof, with most of the access points placed up there. In the next section is the list of all access points and points of interest. The GTA 5 Diamond Casino Heist Scope Out mission has 11 access points in total (seven on the roof, two at ground level, and two more further away from the Casino), along with various points of interest (security cameras or guards that could make it difficult for you when the actual heist takes place).
GTA 5 Diamond Casino Heist Scope Out Casino Guide
