Answer:
In a short way, if an object is moving away, the light that comes from that object will have a red shift (it seems a little more reddish than it actually is)
If the object is moving towards us, the light that comes from the object will have a blue shift (it seems a little more blueish than it actually is)
Those shifts can be observed from the Earth, so from that is safe to conclude that the expanding model is actually correct.
Step-by-step explanation: