We are not told what kind of signs and miracles were being done by Stephen. Presumably they were done as he went about his task of helping with the distribution of food. The fact that the signs are done among the people suggests that they were done in public where they could be seen by more than one person. In this case the miracles performed by Stephen are similar to those done by the Twelve (Acts 2:43; Acts 5:12).1
8 And Stephen, full of grace and power, was doing great wonders and signs among the people.