style.scss

.responsiveCheckerClass {
  float: left;
  display: none;
}

@media only screen and (max-width: 699px) {
  /* dummy-Klasse zur Erkennung des responsiven Layouts */
  .responsiveCheckerClass {
    float: none;
    display: none;
  }
}

test.js

if ($('.responsiveCheckerClass').css('float') == 'left') {
    // desktop
} else {
    // mobile
}