How can I know that 2 x-bee are paired before or not?
Determine the exact part number of the XBee modules you are using, and then determine whether that part number corresponds to a "Series 1" module (e.g., IEEE 802.15.4) or a "Series 2" (a.k.a. ZigBee) module.
Visit Digi International's XBEE/RF SOLUTIONS website and download the reference manuals (PDF files) for the specific XBee modules you are using.
Numerous "quick start guides" are available on the Internet pertaining to XBee "Series 1" and "Series 2" modules. For example, see Exploring XBees and XCTU on the SparkFun Electronics website, or this step-by-step guide titled Basic XBee ZB ZigBee (Series 2) Chat on Digi International's website.


Optional but strongly recommended: Purchase at least one (preferably two) XBee breakout boards (a.k.a., XBee explorer boards) with a built-in USB connector so that you can connect your XBee module to a USB port on your computer. Various companies sell these breakout boards (perform an Internet search using the keywords "xbee breakout usb" or "xbee explorer usb"). In addition, you'll want to download and install on your computer the XCTU software from Digi International's website. Note that in order to USE the XCTU software on a PC running Windows 7 (or later) you must log in as a user who has administrator privileges.

:: CAUTION ::

One final cautionary note. All XBee modules operate off a 3.3&nbsp;VDC voltage supply, and therefore they operate using 3.3&nbsp;Volt logic signals. NEVER connect a 5&nbsp;Volt logic output signal to an XBee 3.3&nbsp;Volt logic input; doing so will damage the XBee module. Companies like SparkFun Electronics, Adafruit Industries, etc. sell bi-directional logic level converter boards if you need to connect, say, a digital output signal from an Arduino Uno (5&nbsp;Volt logic) to an XBee module (3.3&nbsp;Volt logic), or vice versa.