The first Basic Principle in the rules reads:

"Competitors in the sport of sailing are governed by a body of rules that they are expected to follow and enforce. A fundamental principle of sportsmanship is that when competitors break a rule they will promptly take a penalty, which may be to retire."

In a hypothetical close crossing, Boat P (port) is expected to keep clear (rule 10).

If P fails in that obligation, then Boat S (starboard) is expected to protest (see rules 60 and 61).

If P honestly believes that she did not foul S, then P may sail on. Otherwise she is expected to take a penalty (see rule 44).

The rules do not place a burden of proof on either boat. A protest committee must find facts and act based on them. If S took avoiding action, then she must have had a genuine and reasonable apprehension of collision. If so, then P broke rule 10. If not, then P didn't. See ISAF Case 50.

I hope that helps,
Eric