To put it bluntly, without extracting a chunk of muscle fibre for biopsy it’s difficult to test for this. There are gym-based tests that can be done, which require you to complete as many reps as possible of a given exercise. The results then determine if you have a predominance of either muscle fibre type.
For triathletes, we’d make the exercise a leg-based test, however these types of tests are designed more for experienced gym users.
It’s widely agreed that the majority of us are born with an equal balance of fibre types. What’s not agreed is the ability to adapt your fibres. As triathletes, we’d prefer a greater proportion of slow-twitch fibres that are able to sustain moderate levels of activity for long periods. However some research states that we may be able to change by up to 10%.
If you wanted to adapt a greater volume of slow twitch then you’d need to increase the amount of steady, longer sessions, so cut down on sprinting and high-intensity intervals. The majority of training for triathlon is slow-twitch based, so the good news is it will happen naturally. But remember, if you’re not born with Tim Don’s genetics, then it’s difficult to be a world-beating triathlete!