I've talked about my time as a Dr. Squat disciple in the past, and what I learned by reading his material was that fiber recruitment and type-specific training both are affected by load AND the movement involved - but not necessarily the rep count involved. For instance, if you're doing bench press and exploding the bar through the movement, you'll be hitting fast-twitch fibers for sure - and if the load is considerable, you'll recruit fast and slow-twitch alike to be able to handle the load.
However, if you do a workout specifically with high reps and light to moderate loads with slow movements, you'll hit the slow-twitch fibers first, and as you fatigue, you'll recruit whatever fibers you can to complete the reps. One of the workouts I remember was an attempt to completely decimate both types of fibers. You did a 5 x 5 (heavy load) with explosive movement, a 3 x 12 (moderate load) with rhythmic movement, and a 3 x 40 (light load) SLOW. By slow - it was a 2-3 count on the concentric part of each exercise, and then regular speed on the eccentric portion of the movement. The thought was that by paying attention not only to the load involved but also the movement type - explosive, rhythmic, or slow - you could effectively train both fast and slow twitch fibers.
To your original question though, bob - other than an MRI to determine fiber content, I'm not sure of a test to see what percentages of fast and slow twitch fibers you have in each muscle group. I knew a trainer once who claimed to be able to do so using a rather lengthy workout using all kinds of volume - but that just didn't make sense to me. At the end of the day, as everybody here has already said - fibers will be recruited as necessary to move the load, bottom line. Dr. Squat put all kinds of research behind his assertions - so if you want to give it a shot, try altering the way that you do your reps and see if you notice a difference. I can honestly say that I still use many of the things I learned in that vain, and have had great success.
Sorry for the rambling - I hope that helps.