how to identify fake IP or MAC address

i wish to know when we receive packets from the network in case some one is trying to hack your PC and they provide a fake IP or MAC address..

how do we identify that..??

how do we differentiate between real and fake mac address..??

nd i m talking abt d work network whr so many ppl cn try nd hack PCs so as 2 get sm imp info..

pls rply..!!