It is an urban legend, just make believe...some say that bthe PC saying "I sense the darkspawn", that only happen if choosing "Sultry/Cocky" voice and it appear random, even BEFORE the PC taking the Joining
Alistair claim he can sense them that's why Duncan send him with the recruits in Korcari Wild, but the truth is he don't sense any, not even mention if there are darkspawn nearby and the party got ambushed many times
Duncan and the Wardens also failed to know the Tower of Ishal already overrun by darkspawns. Keep in mind that there are a lot of Wardens there before the battle.
Both the PC and Alistair don't sense darkspawns to the end in DA:O
In DA2 however Anders do mention sensing darkspawns in the Deep Road, but the dialogue appear randomly, sometimes not appear at all maybe because of bug