Question: From a British perspective in both WW1 and WW2 the USA have entered the war at a later stage and supported the allies.

The decision for the US to enter the war has often been portrayed to be based on particular acts of aggression against them. The sinking of the Lusitania and the attack on Pearl Harbour.

Was there ever any doubt which side the USA would join? Is there evidence to suggest that these triggers decide which side the USA would join as well as the level of their involvement?

Answer: 
No, the side we would take if we were to enter both wars was undisputed. The only problems was the absence of public unity about going to war. This was a primary concern when America entered the war on April 6, 1917. In Washington, unwavering public support was considered to be crucial to the entire wartime effort. That is why on April 13, 1917, President Wilson created the Committee on Public Information (CPI) to promote the war domestically while publicizing American war aims abroad. Walter Lipmann and Bernays were crucial on this front. 

accessible article <http://www.propagandacritic.com/articles/ww1.cpi.html>. Lipmann's Public Opinion excellent source to see the philosophy behind it.
Alfred Thayer Mahan's doctrine pretty much dictated that the United States will be fighting alongside the British, who were the other naval superpower, rather than Germany which was a land power.

Japan is an interesting case, while at the start of WWII it demolished the British navy, raw racism precluded any possibility of alliance. Recall Japanese immigrants were subject to all kinds of exclusions in the United States at the time, and the implementation of internment.

In any case, the United States wasn't about to share the Pacific anyway.