Prior to modern means of measuring solar output nobody would be able to know if it really changed, absent something like being able to probe the equivalent of meteorological artifacts on other planets. I wonder if lurking somewhere on, say, Saturn is evidence like this. That might show that oh, Saturn was having a (relative) ice age while Earth was having one too. That would point a finger right at Old Sol.
There was one thing they could measure, and that was the sunspot activity. The very fact that we know the records of no sunspot activity between 1645 and 1715 perfect matched the records of very cold winters show that there was already a way to determine how bad winters could be anyway. That’s why some of the highest rainfall totals in California with the last 50 years came during periods of low sunspot activity, which meant a cooler upper atmosphere in winter, allowing the jet stream to travel further south during the winter months.