how do i know if I have a network card?

There are dozens of Mac models and hundreds of variants. We need to know more about your computer before we can tell you who to look for the wifi card. Lacking any other understanding of models and sub-models, the easiest thing to do is do enter the serial number in this Apple suupor page:

https://selfsolve.apple.com/agreementWarrantyDynamic.do

It should return a model description of your computer. If yours is too old for the Apple page to give an answer, this site will do the same thing but works better with older units:

http://www.everymac.com/ultimate-mac-lookup/

Tell us all that is reported for your model EXCEPT the serial number. Also do "About this Mac" from your Apple menu (left end of menubar) and tell us what that shows fro your Mac OS version.

I believe the following is the most likely model you have

http://www.everymac.com/systems/apple/mac_pro/specs/mac-pro-quad-core-2.8-2008-s pecs.html

As you will see the AirPort (wireless) card is/was an option on that model. It did have two built-in Ethernet ports. It is possible to either use a USB WiFi adapter, PCI WiFi card, or buy and fit an original secondhand Apple card. The first choice would be the easiest.

Not sure which model of Mac you got, though every Mac has wired Ethernet hardware, some of Mac, especially note Macs have WiFi.

Check your Mac with Apple Menu > About Mac > System Report > Network.

Two ways I believe. iFixit will tell you as they do teardowns and you can physically look for the part, should be easy without actually tearing down.

 > About This Mac > System Report > Network > Wifi.......

The hardware should be detected and show up here.

If you get a USB wifi adopter or PCle adopter just make sure that it is compatible with the OSX version you are/want to run. Not all adopters are compatible with Yosemite.