The only true way to tell is through a muscle biopsy. Short of that, there are too many vaiables for the lift test: training age, specific lift, technique (or lack thereof), and effort level of the individual, to name a few.
People read too much into the scientific studies and the training practices/ testimonials of elite strength coaches. Those are usually laboratory settings with untrained/ moderately trained individuals or (in the examples of the strength coaches) ELITE level athletes.
The easiest test is the “visual test.” Want to see a TRUE fast twitch animal? Tune in to the 100m sprint in the Olympics and then take a look in the mirror. Unless you look like those guys, chances are you fall into the average category. Not that you truly have average, below average, or above average genetics- it’s all ONE BIG CATEGORY.
Will working in lower rep ranges work for you? Of course- EVERYTHING WORKS (just some of the time). People tend to forget that the simple rules are more important: Lift with proper technique, get stronger on the main lifts, get enough rest, make sure your nutrition is in check, and BUST YOUR ASS when you train. Often times one (or more) of those elements is lacking and a lot of people go searching for “THE THING” thats going to push them to the next level- whether it be a special diet program, the HOTTEST new training program, new supplement, chemical enhancement, whatever.
I wouldn’t pay the fast vs slow twitch thing any more attention than is necessary- and that isn’t much. Screw what the rep test tells you- lift heavy for a while. Do low (3-6 reps) and high sets and see how your body reacts. Assuming you’re doing all the other stuff right, you should see some results. Worst case scenario: you learn a little more about how your body reacts to specific types of training and you become a better coach for the one athlete you really need to worry about- yourself.