Perhaps the most natural measure of similarity between two close languages is the ease with which a native speaker of one can understand the other. (This might not be symmetrical in some cases, because of the background degree of exposure or other reasons.) For less-similar languages, the average time needed to learn the other could be assessed.
More-objective measures could be based on similarity between corresponding words and perhaps grammatical features, while possibly other types of measure have been devised.
My question is as in the title: What measures are there of similarity between languages? And where can I find data on such measurements?