The Roman soldiers would surely have known whether a crucified man was dead, especially since their own lives depended on it. How can this be explained?
The soldiers were not doctors, and they were under the influence of men who were well disposed towards Jesus (as). Even Pilate was surprised that Jesus (as) was supposedly dead already.
The soldiers at the place of execution were not doctors. A man hanging unconscious on a cross after hours of agony can look dead to a layman. They could only judge from the outside.
More important, however, is the situation in which they acted. Pilate, the Roman governor, did not really want to condemn Jesus (as). He washed his hands of the matter and passed the sentence only under heavy pressure from an angry crowd. Joseph of Arimathea, a respected man with good access to Pilate, was well disposed towards Jesus (as). It is quite possible that the soldiers had been told to treat this man gently.
The Gospels themselves give a clue. When Joseph came to Pilate and asked for the body, Pilate was surprised that Jesus (as) was supposedly dead already. Crucified men usually died only after a long time, but Jesus (as) hung on the cross for just a few hours.
Then there is what happened when his side was pierced. Blood and water flowed out separately. That does not come from a dead body, for there the blood no longer flows. It is a sign that the heart was still working.
So one reads the same accounts and reaches a different conclusion. The enemies of Jesus (as) planned his death, but God had another plan. The Holy Qur’an says clearly that he only appeared to people like one crucified.