You use both the Sensing and iNtuition mental muscles.
If you look at the descriptions on this slide, you will see that you use all of these behaviours at some time or other. However, you will tend to use certain behaviours more than others.
Your inborn preferences influence which of these behaviours you use - but so do your upbringing, your training, social/cultural expectations and your employment requirements. You might find it difficult, therefore, to work out whether your inborn preference is for S or for N. This is because (a) you use both S and N and (b) cultural, social and employment expectations, together with your upbringing and training, have also influenced your behaviour.
Bad Joke: Robert F Kennedy once said: "My views on birth control are somewhat distorted by the fact that I was seventh of nine children"