The United States has responded to China claiming sovereignty over South China Sea by contesting those claims by military flyovers and bypasses by military vessels.
China probably could not fire at the planes without escalating the conlict to the point where there would be a major risk of war. Warning shots would carry the risk of being misinterpreted, and probably be ignored anyway, since the explicit purpose is to defy the Chinese claims.
If China instead chose to respond in a tit-for-tat manner to the perceived sovereignty violation and flied through US airspace, possibly somewhere near Guam, but in a way designed to be as unthreatening as possible – for example, by flying with obviously unarmed planes, communicating their intentions to the US and staying away from military installations, only to make a point – what would likely happen? Is it a given that a capable sovereign nation would in such a situation as a last resort shoot the offending plane down, essentially escalating to the same point where the crisis would be if China shot down a US plane?