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.
The spots could reflect upon and/or force climate producing conditions in a number of ways.
But the sun itself radiating more or less (and perhaps in certain bands), is something that the technology hasn’t been there to quantify except the last century or so.
Good science would be following up as vigorously as possible. The sun flickering (on a centuries scale) would be huge news and something that really is worth keeping an eye upon.