How do ICBM early warning systems differentiate between a real threat and a launch of a normal rocket intended to go to space?
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The early stages of a rocket launch is identical to an ICBM launch. It actually used to be the same rockets and a few are still dual use. But an ICBM shuts down its engines when it is still on a suborbital flight. This usually happens before ten minutes into the launch. An orbital launch however keep the engines on until it reaches an orbit. In some cases there is just seconds in difference. But this does indeed mean that you have to wait about ten minutes before you can be sure if it is an orbital launch or an ICBM launch. And by that time it might be too late to stop all warheads.
Another less reliable indicator is the heading of the rocket. Due to the speed you can not really change the direction of a rocket once it is fired so an ICBM have to be fired towards its target from the start. Orbital rockets can be launched into any number of orbits but they tend to be fired either straight east or straight north or south. So even before ten minutes you can get an idea of what the rocket is based on where it is heading.
It is also unlikely that an ICBM attack only include one launch. This would do limited damage and the rocket defenses would not need much advance notice to shoot the few warheads down. So a single rocket launch is likely not an ICBM but if they see more launches at the same time it is more likely to be an ICBM attack rather then an orbital launch.
But the most important indicator is that rocket launches are planned out well in advanced and is therefore published long before it happens. The people manning the early warning systems is therefore briefed about the launch and know what to expect. They still monitor the launch and look for any deviations but their assumption is that it is an orbital launch and not an ICBM.
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