Female relatives of affected males may be carriers. The mothers of affected males, in families with more than one affected male, are carriers. The mothers of affected males with no affected relatives are not always carriers, because their sons may have been affected by new mutations. Sometimes tests will give a definite answer. Sometimes the best that can be achieved is a statement about the probability of carrier status.
Certainty is better, but probabilities are still very useful.
Often the probability that a woman is a carrier is so low that the risk of muscular dystrophy to her baby is much lower than the average risk of serious birth defect or genetic disease. If a woman has an affected son and another affected relative, it is certain that she is a carrier. If she has two affected sons and no other affected relatives, it is very close to certain that she is a carrier. If a woman is a possible carrier, e.g. the sister or aunt or cousin of an affected male, a consistency abnormal SCK test, repeatedly on blood collected under proper conditions, makes it almost certain that he is a carrier. Finally, DNA tests sometimes demonstrate carrier status with certainty or high probability.