You can't verify, the rail has curves and other stuff that block sight.
Also stopping distance for trains is huge. This video for high speed trains says 5km (3mi) https://www.facebook.com/Renfe/videos/%EF%B8%8Fcu%C3%A1l-es-... That is probably at full speed, but IIUC the accident was at half speed so the distance should be like 1.2km (0.8mi).
The driver tried to verify by asking a confirmatory question from the controller. So clearly there are in fact ways to verify, even if we can not achieve 100% certainty.
The driver also could have refused the order to drive at normal speed, and instead follow the protocol which caps speed to 40km/h in situations like these.