So, I have to ask: How can one determine their ratio of fast twitch to slow twitch muscle fibers?
You cannot. It's impossible. The ratio is not constant throughout your whole body, but varies from muscle to muscle and site to site, so even a muscle biopsy cannot give you an answer. It's worthless information anyway.
True, also the size of the various muscle fiber varies according to training, which begs the question when measuring the ratio, do we measure the amount of each fiber or the volume. Fast twich fibers can be increased considerably in size (hypertrophy) whilst slow twitch cannot.
Also there is the problem of fiber type classification, what are the differences between different fibers?