when Rosalind faints at the sight of blood,Oliver suggests her to act like a man for some more time.Do we have to understand that Oliver could sense that Ganemede is a girl?

Most critics agree that Oliver suspects that there is something up with Ganemede. His suspicions are never stated outright. Elizabethans found the whole cross-dressing thing amusing to a point. After a suitable period, they demanded that the genders return to their "natural" state. They felt that the natural order of things must be constant.