ionic-framework/scss/ionic/_radio.scss

// Radio Buttons
// -------------------------------

/* hide a radio button's icon by default */
.radio-content {
  [class^="icon-"],
  [class*=" icon-"] {
    display: none;
  }
}

.radio-item input {
  /* hide any radio button inputs elements (the ugly circles) */
  display: none;

  /* when this radio-item is checked */
  &:checked + .radio-content {

    /* show the radio-content icon when checked */
    [class^="icon-"],
    [class*=" icon-"] {
      position: absolute;
      top: 16px;
      right: $content-padding;
      display: inline;
    }
  }
}