Are there ways to measure how much of the traffic is actually due to reddit being out. I bet there are other reasons for the growth in traffic, like the fact that HN has been making a great job at covering SOPA/PIPA for the past few weeks, I wouldn't be surprised if part of the increase is due to natural SEO. I'm sure one could find other reasons too.

I'm not doubting pg's claim. I'm simply interested and curious: What are the tools available for the analyst/engineer/community manager to analyze the increase?