Thanks to my friend Brett's Airminded blog, I was introduced to Beachcombing's Bizzare History Blog, the first winning entry for interest, which starts with "Food is dangerous at the best of times" has the following quote lifted from it:
Of course, fires are always difficult to interpret in archaeology: there are so many possible culprits including saboteurs, warriors, lightning storms, a drunk servant, a lava stream…You want to read on, don't you? Well here you go; Great Balls of Floury Fire. Don't say I didn't warn you!
If there is no other evidence the best archaeologists can do is talk about ashes and look for signs of rebuilding.