You cannot flag down cabs in Las Vegas the way you can in New York City.
If you want a cab, you must go to a taxi stand (available at every hotel) or call one. Keep the phone number of the Taxicab Authority (702-668-4000) on you when in Vegas, in case your cab driver tries to longhaul you (rip you off).
The most common ripoff is when cab drivers take visitors going from McCarran Airport to the Strip via the Tunnel. They are required to ask you first if they can take the tunnel; if they don't, chances are good they're longhauling you. 99.9% of the time, there is no good reason to take the tunnel, and it just adds onto your expense. (The exception is if you're traveling south of the airport.)
To avoid being longhauled, when you get in the cab, simply tell the driver where you want to go and add "No Tunnel." It also helps to familiarize yourself with the streets between points you are traveling between (I've also heard of cab drivers longhauling people traveling between the Strip and Downtown and other points in Vegas, too. Unfortunately, I've learned the hard way that the cab drivers who really want to longhaul you will come up with an excuse not to take the route you suggest (usually "Oh there's construction on that route, it'll take a long time"). If you suspect you've been longhauled, you can seek assistance from the valet at your hotel when you arrive--before you pay the cabbie, but after you've gotten your luggage out of the cab--or call the Taxicab Authority yourself.
Most of the cab drivers I've dealt with in Las Vegas were honest, but you need to be aware that the crooks do exist and be prepared for them.
It’s also advisable to get an estimate of the fares for specific routes before your trip. That information is available at the Taxicab Authority website.
Written and contributed by Gray Cargill