The default theme (as of May 1st 2015) is based on Bootstrap. If you created your form after that date and have not changed the theme, then it is using Bootstrap. It is also easy to identify a Bootstrap theme from the larger, cleaner fields with solid borders like this:
Whereas fields on older themes have an inset border and are smaller:
In general, the Bootstrap style themes have more solid color, larger fonts and a more modern and clean look.