In Wiki for Kosmos 954 there is:
In mid-December North American Aerospace Defense Command, which had assigned the satellite the Satellite Catalog Number 10361, noticed Kosmos 954 making erratic manoeuvres, changing the altitude of its orbit by up to 50 miles, as its Soviet operators struggled to control their failing spacecraft.
This is great mental shortcut which I don't understand. How did NORAD noticed Kosmos 954 making erratic manoeuvres? By what technical means?
This is similar quesiton to "How do you recognise an island?"