Test if a browser supports a CSS selector

Really simple: how do I most accurately test if a browser has support for a certain CSS selector?

I currently have some CSS code that makes the page a little more interactive by using the :checked selector in CSS, but I want to create a fallback script that does the same thing with JavaScript, but only if the user's browser has no support for the :checked selector.

My question is, how do I most accurately test if the user's browser supports a certain CSS selector?

Here is the code I'd like to use it on:

HTML:

<label class="coolbox">
    <input type="checkbox"/>
    <span>I want to eat some caek.</span>
</label>

CSS:

.coolbox input {display:none;}
.coolbox span::before {
    content: "";
    display:inline-block;
    width:10px;
    height:10px;
    margin-right:5px;
    border:1px solid black;
}
.coolbox:hover span::before {border:1px solid #555;}
.coolbox:active span::before {border:1px solid #999;}

.coolbox span::before {background-color:#F77;}
.coolbox input:checked + span::before {background-color:#4A4;}

Demo

PS: I'd prefer not to just use conditional comments, because I'd like to follow the standard of detecting features instead of browsers.

Workaround for your case:

<input type=checkbox checked=checked>

css:

input{
  font-family:'arial';
}
input:checked{
  font-family:'sans-serif';
}

checking procedure: js

alert($('input').css('font-family')=='sans-serif'?'supported':'not supported');