This is a tricky one, what you need to do is go up a very tall building with both fermenters. Before starting take a weight reading of both fermenters while atop the building, this is your original gravity. Subtract the weight of any foam obviously.
Firstly lob the ale fermenters off the building, and time its fall, from throw until impact, this is your terminal gravity. Do the same for the lager fermenter. Multiply the original gravity by the terminal gravity and divide the result by 6.44 for lager and 7.11 for the ale. This will tell you which is faster.