The Romans would have never allowed Him to be taken down from the cross if He were not dead. Furthermore, the Jews would have never buried Him if He were not dead.
We have things we don’t know. The most important thing is Jesus’ message.
The Romans were master executioners. When they crucified someone they ensured they were dead before they were removed from the cross. Those who were found still breathing after so many agonizing hours and sometimes days had their legs broken prior to being taken down so they could no longer lift themselves up to breathe (see John 19:31-34). It was a cruel and horrendous method. The Bible says that they did not break Jesus' legs because He was already dead. A spear thrust into His side resulted in water and blood pouring out - a sign that they had ruptured His pericardial sack and He was confirmed dead.
Also, Jesus told His disciples many times that He would die and then be raised from the dead after three days. I've heard the skeptic opinions about Jesus possibly not dying but reviving once he was placed into the cool, dark tomb. They don't realize that He endured terrible torture before He was nailed to the cross and lost a lot of blood . Those who removed Him and washed Him prior to placing Him in the tomb also would not have wrapped Him in burial cloths had He still been breathing. I've read that burial practices involved a weighty amount of spices and linen cloths. Not at all conducive to recovering from severe bleeding, beatings, flesh-tearing whipping, a crown of thorns dug into His head, suspension from a tall crossbeam using ropes and large iron nails driven into both the wrist and foot joints. Like I said - an unbelievably cruel way of killing people.