How to fire an event when a radio-button (input) is checked?

I have two type="radio" inputs with the first option being checked by default. What I'd like to achieve is to fire a particular event if the second option (input) is checked. I've tried to do it though basic 'if/else' statement (detecting of a particular input has 'checked' attribute), but it doesn't work...

Here's the HTML part:

<input type="radio" name="radio-group" id="sendnow" value="Send Now" checked>
<label for="sendnow" class="camp_lbl">Send Now</label>
<input type="radio" name="radio-group" id="sendlater" value="Schedule Send">
<label id="zzz" for="sendlater" class="camp_lbl">Schedule Send</label>

And the CSS:

#send_options_radio_selectors input[type="radio"] {
  position: absolute;
  opacity: 0;
  -moz-opacity: 0;
  -webkit-opacity: 0;
  -o-opacity: 0;
}

#send_options_radio_selectors input[type="radio"] + label {
  position: relative;
  margin-right: 12%;
  font-size: 1.5rem;
  line-height: 23px;
  text-indent: 25px;
  color: #505e6d;
  cursor: pointer;
}

#send_options_radio_selectors input[type="radio"] + label:before {
  content: "";
  display: block;
  position: absolute;
  top: 2px;
  height: 18px;
  width: 18px;
  background: white;
  border: 1px solid gray;
  box-shadow: inset 0 0 0 2px white;
  -webkit-box-shadow: inset 0 0 0 2px white;
  -moz-box-shadow: inset 0 0 0 2px white;
  -o-box-shadow: inset 0 0 0 2px white;
  -webkit-border-radius: 50px;
  -moz-border-radius: 50px;
  -o-border-radius: 50px;
}

#send_options_radio_selectors input[type="radio"]:checked + label:before {
  background: #e16846;
}