Your question is largely answered in the FAQ

Originally our estimates were based on John Adams who wrote that 1/3 supported independence 1/3 supported the crown and 1/3 were uncommitted. Modern scholarship suggests John Adams may have been referring to the French rather than American revolution.

Today our best estimates are Patriots 40-55% Loyalists 15-25% uncommitted 30-45%

We get these numbers by reading personal writings such as journals. People would often indicate events of the day describing things such as tension between neighbors over the issue. Another way is to look at historical events. For example patriots engaged in several incidents of violence against loyalists throughout the war. By comparing the frequency of these events, where they happened, and who was involved we can get a reasonable idea of how the population was divided.