In chapter five of Beowulf, Wulfgar (the herald and marshal of Hrothagar) meets Beowulf and his band of warriors. Upon this initial meeting, Wulfgar immediately states "never have I met so many foreigners of heroic bearing. Methinks that it's for glory—not because of exile, but for courageous valor—that you seek Hrothgar!” Given Wulfgar's immediate reaction to the band of men, something must have triggered this innate response.
Prior to Wulfgar's statement, the text defines Wulfgar as a "proud warrior." Although seemingly unimportant, one could interpret Wulfgar's own existence as a warrior as the qualifier which makes him an expert on the subject of warriors. Essentially, the old cliche "it takes one to know one" comes into play here.
Another suggestion would be that the appearance of the warriors triggers Wulfgar's characterization of the men. He immediately recognizes their "heroic bearing." Heroes are not typically exiled; they are praised.