The mechanism is economical: if an emotion is unacceptable to the image I hold of myself, I move it outside. It is not that I am angry — he is aggressive. It is not that I want to leave — she is thinking of leaving me.
What makes projection hard to spot from inside is that it produces evidence. Once someone has decided a partner is distant, they will scan that partner's behaviour for confirmation, read ambiguity in the expected direction, and respond by withdrawing — which in turn produces real distance. Projection tends to fulfil itself.
It is not necessarily pathological. Everyone does it occasionally, especially under stress, exhaustion or in highly charged relationships. It becomes a problem when it is the usual way of explaining what happens between people.
The most useful practical sign: disproportionate intensity. A trait that irritates beyond all measure in someone else, particularly one most people find unremarkable, is worth asking about as a rejected part of oneself.