How to know if a PC game is licensed genuine original?

There are a few ways to know and check:
Firstly, the original PC / Computer game usually have a unique license called a CDKEY / license key.

The CDKEY is a string of characters and number like some barcode. You need to enter this during the installation of the game. Take for instance Microsoft's operating system like Windows XP and Vista, they come with a license CDKEY as well and you have to do the same during installation. NOTE: Not all games are developed with a CDKEY. Some games do not have this. That is fine.

On the other hand, pirated / fakes / copied games usually have a CRACK code that is required to be copied onto the installed folder in your computer's harddisk after installation.

Else the game will not load. This is a telling sign the game is pirated. The keywords to look for are CRACK and also the main game's EXE file. Once the CRACK is in place, you won't require the CDs to run the game anymore. Compare this to original games, whereby you are almost always required to put the original Play CD onto the CDROM / DVD ROM in order to play the game each and everytime. (NOTE: Not all games does this as well).

Next, most games requires you to place the original CD onto the CDROM drive to play the game.

Even if you've installed the whole game. This is to (sort of) prevent buyers from installing on many computers. Eventually it is annoying to keep popping in the original CD onto 2 or 3 computers. This is quite a major headache for genuine buyers who paid for the license legal game. While the developers argue it's a safety measure. Gamers feel it restricts their gaming experience.

Lastly: newer games have "online authentication" capabilities.

Or online activation. Basically the CDs that come with the game is not really essential. The main factor is the license, and user account. Think for instance, you have a bank account with Swiss Bank. Even if someone stole the bank book, you are still the valid owner because your telephone number is registered to the account. Likewise, the gaming account is registered to your email - and obviously only you have access to the email account. Games like Counter Strike Source, Half Life 2 - uses this system. Also new online multiplayer - massive online multiplayer games like World of Warcraft, Guild Wars, employ the same system. You can "steal someone's installation CDs" but you cannot gain access to play the game.

In the near future, PC games - and in the broader sense - computer software will be distributed on this basis. The buyer just pays online, and gets the license - then starts to install the game online - when internet speeds are super high compared to current internet speeds. Retail game sales will certainly drop in favour of a secure way to distribute games, and reducing cost - virtually.