If YDS or Harvard has their own Director or Assistant Director of Student Services that might be a good person to ask. At least at BC that is the person that helps people find jobs after graduating, so she is often told if people are going to doctoral programs. Also, professors in your specialty are a great resource because they probably wrote recommendations for those people and know how it turned out. If you go to accepted students days, asking the students could also give you a sense of it. Admissions probably has information on it, but I would just be cautious because some Admissions team stretch the truth a bit and make it sound like a lot of people for example get into Yale's PhD program when in reality one person has been accepted in the last seven years or something like that.